Improved the error message that triggers when you accidentally target a directory

[mattpocock]

Fixes #38

[changeset-bot[bot]]

🦋 Changeset is good to go

Latest commit: e03246dc7f6117a343636536e7a0a92d6cad79da

We got this.

This PR includes changesets to release 1 package

| Name | Type | | -------------- | ----- | | xstate-codegen | Patch |

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR