Bug 24008: Display a warning message when deleting a patron with outstanding credits

Bug 24008: Display a warning message when deleting a patron with outstanding credits

So far nothing is displayed if a librarian removes patron with
outstanding credits.
Note that outstanding debits blocks the deletion.

Test plan:
- Create a patron with outstanding credit
- Create a patron with outstanding debits
- Delete the 2 patrons
=> With credit - You get a warning but do not block the deletion
=> With debits - You get a warning message that blocks the deletion

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>