Return non-zero exit code on parse failures

[p3l6]

Currently, when thrift-typescript fails to parse a file, you get the following logs from node: (after the parse error is printed by the parser)

(node:40855) UnhandledPromiseRejectionWarning: Error: Unable to parse source: 
    at parseThriftString (/Users/paul/var/Underwriter/node_modules/@creditkarma/thrift-typescript/dist/main/parser/index.js:11:19)
    at Object.parseThriftFile (/Users/paul/var/Underwriter/node_modules/@creditkarma/thrift-typescript/dist/main/parser/index.js:25:23)
    at /Users/paul/var/Underwriter/node_modules/@creditkarma/thrift-typescript/dist/main/index.js:69:67
    at Array.map (<anonymous>)
    at thriftProjectFromSourceFiles (/Users/paul/var/Underwriter/node_modules/@creditkarma/thrift-typescript/dist/main/index.js:69:37)
    at processThriftProject (/Users/paul/var/Underwriter/node_modules/@creditkarma/thrift-typescript/dist/main/index.js:114:12)
    at async Object.generate (/Users/paul/var/Underwriter/node_modules/@creditkarma/thrift-typescript/dist/main/index.js:118:27)
(node:40855) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:40855) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

This results in an exit code of 0. These failures produced a nonzero exit code, which would be helpful for scripting/chaining/etc.

This PR proposes to catch any otherwise unhandled promise rejections and exit(1) instead of waiting for node to do this in the future.

[p3l6]

Closing due to lack of interest.