Bug 32426: Throw InvalidUserid exception in Patron->store

Bug 32426: Throw InvalidUserid exception in Patron->store

When creating we still call generate_userid and verify the result.
When modifying we do not accept an invalid userid. When needed,
we recreate the userid; this should be very exceptional.

Test plan:
Run t/db_dependent/Koha/Patrons.t
Go to staff interface. Try changing userid of a patron to an
existing one.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>