[events] Better handling of eroneous events.

Summary:
There are a few accounts which have invalid events (e.g. cancelled in
gmail, or unable to get info from icloud). Currently our logs are pretty bloated
and these particular issues are minor. As such, handle these in a less noisy fashion.

Test Plan: regression

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D454