Handle gracefully some IMAP errors

Summary:
Apparently IMAPClient raises a specific exception when it encounters a "transient" error (i.e: an error which may be fixed when retrying).

I've added code to handle this class of errors for the syncback backend and the login code. This should fix T112 and also some errors we see popping up randomly on sentry (ex: https://app.getsentry.com/inbox/beta/group/28209207/)

Test Plan: Can't really test this kind of thing

Reviewers: spang, charles

Reviewed By: charles

Maniphest Tasks: T112

Differential Revision: https://review.inboxapp.com/D317