Bug 28056: Add Koha::Exceptions::Patron::MissingMandatoryExtendedAttribute

Bug 28056: Add Koha::Exceptions::Patron::MissingMandatoryExtendedAttribute

This trivial patch adds a new exception for more fine-grained control on
the errors when dealing with patrons and their extended attributes.

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/Koha/Exceptions.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>