bubble up the error message / improve error message when failing to parse the schema

[BlooJeans]

Two small things:

1. If you fail to load the spec, then return early. Otherwise the swaggerToTS function will attempt to execute and obviously fail, giving you a misleading error message

2. Bubble up the error message returned from yaml.safeLoad() to give you an idea of why the script is failing

Before:

After:

[drwpow]

Thank you! Agree this is an improvement.

[drwpow]

@all-contributors please add @BlooJeans for code

[allcontributors]

@drwpow

I've put up a pull request to add @BlooJeans! 🎉