google: gracefully handle 'member already exists'.
This improves error handling in add_email_to_google_group to:
- No longer create Sentry errors when a user is already in a group. A warning will be logged.
- Only log Sentry error when the 404 is caused by the group not being found, not by a user's email not being a Google account. In the second case only an warning will be logged.
A future improvement would be notify the user in the second case.
closes #1149 (closed)