Clearer error messages, fallback behavior when auth token is invalid

[fennifith]

I didn't realize this when making the project, but the GitHub API will return a 400 error if it is passed an invalid auth token. This causes all requests sent by the library to fail, and any data needed from GitHub will not be fetched.

Ideally, the library should simply resend the request without the auth token in this situation, and should warn the developer that it is invalid. This will make troubleshooting a lot easier, rather than failing to identify the problem and blaming it on something else (prime example: #64).

I should probably also put something in the README about not committing auth tokens to version control.