Bug 27857: (follow-up) Use a more fine-grained exception for mandatory attributes

Bug 27857: (follow-up) Use a more fine-grained exception for mandatory attributes

On using this dev for the API, I found that the original FKConstraint
exception was too generic, and could be confused with those risen by
Koha::Patron->store. So this patch changes the exception that is thrown
in Koha::Patron->extended_attributes

The change is reflected on the test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Patron.t
=> SUCCESS: Tests pass, and they include the new exception
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>