Handle Github Rate-Limit

[theseer]

In case of a 403 error on github's API, we should probably check for a Rate Limit and report a better error message in case it is.

[Ayesh]

👍 for better error message. Perhaps it's better if we expose an option, preferably from a file, to set an OAuth key so the rate limits can be high as 5000 req/hr.

[theseer]

We in general need to add support for means of authentication like for private repositories. That's a different issue though.

[sebastianheuer]

As of 4f9b71f it is possible to set a GitHub Auth Token via the GITHUB_AUTH_TOKEN environment variable. We will probably add other ways to configure this, but environment variables are usually supported by CI tools out of the box, so we figured this was a good first step.