More specific error message when github auth fails

[teodorlu]

Please answer the following questions and leave the below in as part of your PR.

• This PR corresponds to an issue with a clear problem statement.

  • On expired NEIL_GITHUB_TOKEN, neil dep upgrade does nothing, then terminates with exit code 0 #237

• I have updated the CHANGELOG.md file with a description of the addressed issue.

• I have considered whether I should add more tests covering the code I've changed.

  • We don't currently write test that assert the contents of error messages on stdin, and I don't see a good reason to change that practice now.

Proposed solution

Invalid Github token error messages before this PR:

$ neil dep upgrade
{"message":"Bad credentials","documentation_url":"https://docs.github.com/rest","status":"401"}
{"message":"Bad credentials","documentation_url":"https://docs.github.com/rest","status":"401"}

Invalid Github token error messages after this PR:

$ neil-dev dep upgrade
Your neil github token is invalid or expired.
Please double check your  NEIL_GITHUB_TOKEN  environment variable.
See neil's README for more details.
Your neil github token is invalid or expired.
Please double check your  NEIL_GITHUB_TOKEN  environment variable.
See neil's README for more details.

Other changes

• Behavior outside of error messages is left unchanged
• I pulled the environment variables for Github tokens (eg NEIL_GITHUB_TOKEN) into a def in order to provide more specific error messages

Error message formatting

Other error messages are printed with the first line not indented, and following lines indented:

You've hit the github rate-limit (60 reqs/hr).
  You can set an API Token to increase the limit.
  See neil's README for details.

I'm not sure whether this is the desired way to print errors, or if it's accidental, eg code indenting. Clarification welcome!

[borkdude]

Thanks!