Build gets stuck when an image generation finishes early

To reproduce:

Input

Have this on one of the Markdown page for Gatsby:

@startuml
Bob -> Alice
@enduml

Steps

Clean up public and .cache folders
Run npm run develop or npm run build

Actual result

Build hangs with output:
```
$ npm run develop
```

docs-starter@1.0.0 develop /Users/... gatsby develop

success open and validate gatsby-configs - 0.038 s success load plugins - 0.350 s success onPreInit - 0.015 s success initialize cache - 0.020 s success copy gatsby files - 0.068 s success onPreBootstrap - 0.025 s success source and transform nodes - 0.121 s success Add explicit types - 0.021 s success Add inferred types - 0.129 s success Processing types - 0.071 s success building schema - 0.337 s success createPages - 0.038 s success createPagesStatefully - 0.038 s success onPreExtractQueries - 0.014 s success update schema - 0.033 s success extract queries from components - 0.128 s success write out requires - 0.039 s success write out redirect data - 0.014 s warn The icon(src/images/favicon.png) you provided to 'gatsby-plugin-manifest' is not square. The icons we generate will be square and for the best results we recommend you provide a square icon.

success Build manifest and related icons - 0.198 s success onPostBootstrap - 0.222 s ⠀ info bootstrap finished - 4.996 s ⠀ success run static queries - 0.024 s — 2/2 230.48 queries/second warn code block or inline code language not specified in markdown. applying generic code block ⠇ run page queries — 7/8 47.83 queries/second


**Tested with**

- version: "0.1.2"

<details><summary>Details</summary>
<p>

```bash
$ node -v
v10.16.3

$ npm -v
6.9.0

$ java -version
java version "1.8.0_221"
Java(TM) SE Runtime Environment (build 1.8.0_221-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)

$ dot -V
dot - graphviz version 2.40.1 (20161225.0304)

$ grep -A10 gatsby-remark-plantuml package-lock.json 
    "gatsby-remark-plantuml": {
      "version": "0.1.2",
      "resolved": "https://registry.npmjs.org/gatsby-remark-plantuml/-/gatsby-remark-plantuml-0.1.2.tgz",
      "integrity": "sha512-cDvtF6luPnF+jnlgw4WvNnI4CdhODWu8WLr9QRyjauKpyF4rV04FXcv9R3dOMCpermJs1F2RVwCqDaTpmq79bw==",
      "requires": {
        "@babel/runtime": "^7.6.0",
        "@rauschma/stringio": "^1.4.0",
        "bluebird": "^3.5.1",
        "hasbin": "^1.2.3",
        "unist-util-visit": "^1.4.1"
      }

 $ grep -A190 \"gatsby\" package-lock.json 
    "gatsby": {
      "version": "2.15.28",
      "resolved": "https://registry.npmjs.org/gatsby/-/gatsby-2.15.28.tgz",
      "integrity": "sha512-D2TlJaPL9daCKJ7yU1T/BoebZM5uaEQo+2ojZwANgY72l/WIrqTCSXH+lGbaAZJFPdZEIUQfUBlnIWrvNapTzw==",
      "requires": {
        "@babel/code-frame": "^7.5.5",
        "@babel/core": "^7.6.2",
        "@babel/parser": "^7.6.2",
        "@babel/polyfill": "^7.6.0",
        "@babel/runtime": "^7.6.2",
        "@babel/traverse": "^7.6.2",
        "@gatsbyjs/relay-compiler": "2.0.0-printer-fix.4",
        "@hapi/joi": "^15.1.1",
        "@mikaelkristiansson/domready": "^1.0.9",
        "@pieh/friendly-errors-webpack-plugin": "1.7.0-chalk-2",
        "@reach/router": "^1.2.1",
        "@typescript-eslint/eslint-plugin": "^1.13.0",
        "@typescript-eslint/parser": "^1.13.0",
        "address": "1.1.2",
        "autoprefixer": "^9.6.1",
        "axios": "^0.19.0",
        "babel-core": "7.0.0-bridge.0",
        "babel-eslint": "^10.0.3",
        "babel-loader": "^8.0.6",
        "babel-plugin-add-module-exports": "^0.3.3",
        "babel-plugin-dynamic-import-node": "^1.2.0",
        "babel-plugin-remove-graphql-queries": "^2.7.10",
        "babel-preset-gatsby": "^0.2.17",
        "better-opn": "1.0.0",
        "better-queue": "^3.8.10",
        "bluebird": "^3.5.5",
        "browserslist": "3.2.8",
        "cache-manager": "^2.10.0",
        "cache-manager-fs-hash": "^0.0.7",
        "chalk": "^2.4.2",
        "chokidar": "3.1.1",
        "common-tags": "^1.8.0",
        "compression": "^1.7.4",
        "convert-hrtime": "^2.0.0",
        "copyfiles": "^1.2.0",
        "core-js": "^2.6.9",
        "cors": "^2.8.5",
        "css-loader": "^1.0.1",
        "debug": "^3.2.6",
        "del": "^5.1.0",
        "detect-port": "^1.3.0",
        "devcert-san": "^0.3.3",
        "dotenv": "^8.1.0",
        "eslint": "^5.16.0",
        "eslint-config-react-app": "^4.0.1",
        "eslint-loader": "^2.2.1",
        "eslint-plugin-flowtype": "^3.13.0",
        "eslint-plugin-graphql": "^3.1.0",
        "eslint-plugin-import": "^2.18.2",
        "eslint-plugin-jsx-a11y": "^6.2.3",
        "eslint-plugin-react": "^7.14.3",
        "eslint-plugin-react-hooks": "^1.7.0",
        "event-source-polyfill": "^1.0.8",
        "express": "^4.17.1",
        "express-graphql": "^0.9.0",
        "fast-levenshtein": "^2.0.6",
        "file-loader": "^1.1.11",
        "flat": "^4.1.0",
        "fs-exists-cached": "1.0.0",
        "fs-extra": "^8.1.0",
        "gatsby-cli": "^2.7.53",
        "gatsby-core-utils": "^1.0.12",
        "gatsby-graphiql-explorer": "^0.2.21",
        "gatsby-link": "^2.2.17",
        "gatsby-plugin-page-creator": "^2.1.23",
        "gatsby-react-router-scroll": "^2.1.11",
        "gatsby-telemetry": "^1.1.28",
        "glob": "^7.1.4",
        "got": "8.3.2",
        "graphql": "^14.5.8",
        "graphql-compose": "^6.3.5",
        "graphql-playground-middleware-express": "^1.7.12",
        "invariant": "^2.2.4",
        "is-relative": "^1.0.0",
        "is-relative-url": "^3.0.0",
        "is-wsl": "^2.1.1",
        "jest-worker": "^24.9.0",
        "json-loader": "^0.5.7",
        "json-stringify-safe": "^5.0.1",
        "lodash": "^4.17.15",
        "lokijs": "^1.5.7",
        "md5": "^2.2.1",
        "md5-file": "^3.2.3",
        "micromatch": "^3.1.10",
        "mime": "^2.4.4",
        "mini-css-extract-plugin": "^0.8.0",
        "mitt": "^1.1.3",
        "mkdirp": "^0.5.1",
        "moment": "^2.24.0",
        "name-all-modules-plugin": "^1.0.1",
        "normalize-path": "^2.1.1",
        "null-loader": "^0.1.1",
        "opentracing": "^0.14.4",
        "optimize-css-assets-webpack-plugin": "^5.0.3",
        "parseurl": "^1.3.3",
        "physical-cpu-count": "^2.0.0",
        "pnp-webpack-plugin": "^1.5.0",
        "postcss-flexbugs-fixes": "^3.3.1",
        "postcss-loader": "^2.1.6",
        "prompts": "^2.2.1",
        "prop-types": "^15.7.2",
        "raw-loader": "^0.5.1",
        "react-dev-utils": "^4.2.3",
        "react-error-overlay": "^3.0.0",
        "react-hot-loader": "^4.12.14",
        "redux": "^4.0.4",
        "redux-thunk": "^2.3.0",
        "semver": "^5.7.1",
        "shallow-compare": "^1.2.2",
        "sift": "^5.1.0",
        "signal-exit": "^3.0.2",
        "slash": "^3.0.0",
        "socket.io": "^2.3.0",
        "stack-trace": "^0.0.10",
        "string-similarity": "^1.2.2",
        "style-loader": "^0.23.1",
        "terser-webpack-plugin": "1.4.1",
        "true-case-path": "^2.2.1",
        "type-of": "^2.0.1",
        "url-loader": "^1.1.2",
        "util.promisify": "^1.0.0",
        "uuid": "^3.3.3",
        "v8-compile-cache": "^1.1.2",
        "webpack": "~4.41.0",
        "webpack-dev-middleware": "^3.7.1",
        "webpack-dev-server": "^3.8.1",
        "webpack-hot-middleware": "^2.25.0",
        "webpack-merge": "^4.2.2",
        "webpack-stats-plugin": "^0.3.0",
        "xstate": "^4.6.7",
        "yaml-loader": "^0.5.0"
      },

MacBook Pro (15-inch, 2019) 2,3 GHz Intel Core i9 16 GB 2400 MHz DDR4 Radeon Pro 560X 4 GB Intel UHD Graphics 630 1536 MB

MacOS X 10.14.6 (18G95)

baerrach / gatsby-remark-plantuml

Build gets stuck when an image generation finishes early #2