[dwilding]

Currently, if an invalid token is provided in GIMMEGIT_GITHUB_TOKEN, gimmegit crashes:

Getting repo details
Traceback (most recent call last):
  File "/home/me/.local/bin/gimmegit", line 10, in <module>
    sys.exit(main())
             ~~~~^^
...
    raise self.createException(status, responseHeaders, data)
github.GithubException.BadCredentialsException: 401 {"message": "Bad credentials", "documentation_url": "https://docs.github.com/rest", "status": "401"}

This PR catches the error and outputs a friendly message:

Error: GitHub rejected the token. Has the token expired?