bug 809607: Abort calendar sync event processing if account is deleted during the process

ATTN: @lightsofapollo - Okay, so I think I've finally absorbed enough of how the sync process works to try fixing this issue. This seems to solve it for me on my phone. 

Not entirely sure how to stop the sync process (or if that's possible), but this PR makes `#commit` a no-op and causes the rest of the events to be ignored if account removal is detected in mid-sync.