Exit with error status codes in error scenarios

[Samstiles]

I'd like to find the time one weekend to PR this myself, but adding it to GH issues for reference sake:

In an error scenario, the hogan CLI should exit with a non-zero status code to denote the error for consumers of the CLI.

For example if I am using hogan to generate configs as a part of my jenkins pipeline, I do not have an easy way to know whether or not the hogan config generation attempt failed or succeeded in my scripting.

Two scenarios I know of right now:

• invalid hogan config dir specified
• no matching environment in the configs dir... I do think this should produce an error