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

[mbrukman]

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 or another doc about adding an email address to one's GitHub account.