ItemSync/TransactionSquence: Don't abort on error

The ItemSync must never emit result before it's complete. This patch fixes
this behvaiour in case of an error on the ItemCreateJob.

* Adding a job does not reset the status if a rollback was initiated already,
and thus avoids a second rollback on commit() after one was already performed
earlier (this resulted in the "No transaction in progress" error message).
* The rollback job, that is equally added using addSubjob still gets regularly
executed.

REVIEW: 120527