Bug 26633: (QA follow-up) Tests on error condition

Bug 26633: (QA follow-up) Tests on error condition

This patch highlights a problem with the try/catch conditions in the
ad() controlled method. Tests are added, and the controller is fixed to
reflect the right behaviour.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/transfer_limits.t
=> SUCCESS: Tests pass! 409 is returned as appropriate!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>