fixes https://github.com/TryGhost/Team/issues/2332
Saves events in the database and collects error information.
Do note that we can emit the same events multiple times, and as a result
out of order. That means we should correctly handle that a delivered
event might be fired after a permanent failure. So a delivered event is
ignored if the email is already marked as failed. Also delivered_at is
reset to null when we receive a permanent failure.
d8187123af