Bug 21337: (QA follow-up) Rollback for partial delete

Bug 21337: (QA follow-up) Rollback for partial delete

Puts delete loop in a txn_do.
Raises Koha::Exceptions::Patron::Delete when Patron->delete does not return
true (like 0 or -1).
Unit test adjusted accordingly.

Note: A follow-up report for raising exceptions in Object->delete could well
be considered. Not here please.

Test plan:
Run t/db_dependent/Koha/Patrons.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>