Bug 17208: Checking if classification source or filing rule already exists before...

Bug 17208: Checking if classification source or filing rule already exists before adding

To test:
1) Go to Admin -> Classification sources
2) Attempt to reproduce bug before applying patch. Notice the message
saying the add was successful, but was not added to the table
3) Apply patch and refresh page (restart plack if necessary)
4) Add new classification source with same code as existing one. Notice
you are now told that the add failed.
5) Confirm adding new classification source with unique code works
6) Add new classification filing rule with same code as existing one.
Notice you are told that add failed.
7) Confirm adding new classification filing rule with unique code works

Sponsored-by: Catalyst IT
Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
This is not the way to go, we should use an eval instead.
But since we do not have RaiseError set, it will not work.
This module will need to be moved to Koha::Objects to be implemented
correctly.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>