Bug 26555: Add ->messages and ->add_message to Koha::Object

Bug 26555: Add ->messages and ->add_message to Koha::Object

This patch adds a way to make Koha::Object-derived classes to carry
messages around. This targets non-fatal errors, or around action flags
for the caller, to avoid complex exception handling on the controllers when
it is not expected to be fatal.

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

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>