Bug 21022: (follow-up) Move overloaded full_message method to subclass

Bug 21022: (follow-up) Move overloaded full_message method to subclass

We shouldn't have it overloaded on the base class, as it could get huge
and difficult to find things, and read.

This patch moves things to Koha::Exceptions::Object. We should overload
the full_message method on each exceptions class, as needed.

To test:
- Run:
  $ kshell
 k$ prove t/Koha/Exceptions.t
=> SUCCESS: Tests still pass!
- Sign off :-D

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