Continue syncing calendar after failure

[a_user]

Steps to reproduce:

1. Set up a Caldav calendar (mailbox.org here).
2. With DAVx5 connect to this calendar.
3. Cause an syncing error. In this case deleting a recurrent birthday reminder caused an error 403 (Forbidden) from the server.

Expected behaviour:

1. DAVx5 continues with the next sync item, trying to sync as many items as possible.
2. DAVx5 informs the user about all failed items.

Actual behaviour:
DAVx5 stops the sync. Many other sync items are not synced because one single failure.