[sendmail/client] Better error handling / logging when errors occur while trying to send mail.~

Summary:
OAuthError bubbles up to top if we can't login to the account during
sendmail. Instead, catch that exception and turn it into a generic sendmail
exception with proper logging. Note: exception still bubbles up to top, need
more generic way of handling these issues (i.e. keep retrying and/or eventually
fail), however that needs to be done across all actions.

Fixes T479

Test Plan: review code / regression

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T479

Differential Revision: https://review.inboxapp.com/D501