exit / fail loudly on invalid versions to yarn gen

[ghost]

As a contributor I would like to know if I am using the cli incorrectly. Without validation up front I think I am passing correct parameters when I am not. This has been part of my experience in #74 .

I have been trying to generate ember-data API documentation using:

 yarn gen --project ember-data --version 3.14

This is incorrect.
I should spell out the version explicitly:

 yarn gen --project ember-data --version 3.14.0

Without the exact version specified the tmp/rev-index folder is not populated correctly and so ember-api-docs app fails to find the payloads.