Provide more specific error messages for missing email ↔ GitHub account correspondence

E.g., when the user's GitHub id is empty (typically due to email address not being in the user's profile), CRBot should specify that, instead of failing with a generic error such as:

> Please verify the committer name, email, and GitHub username association are all correct and match CLA records.

This could also link to [these docs](https://help.github.com/en/github/committing-changes-to-your-project/why-are-my-commits-linked-to-the-wrong-user) or another doc about adding an email address to one's GitHub account.