IMAP: do not leave lingering tasks behind when mailbox syncing fails

When the ObtainSynchronizedMailboxTask failed, the waiting KeepMailboxOpenTask
would not unregister itself from the Parser's maintainingTask, and therefore
further tasks kept getting queued "behind" this KeepMailboxOpenTask which stuck
for all practical purposes.

This does not fix #320713 (no UI popup is shown), but at least it makes sure
that further activity can be resumed. A proper indication should be added,
definitely.