Bug 26515: Add Koha::Acquisition::Order->cancel

Bug 26515: Add Koha::Acquisition::Order->cancel

This patch introduces an OO replacement for DelOrder. It does the same
thing. It doesn't die when trying to delete items or biblio. It sets an
error message on the order object so the caller knows what happened.

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Order.t
=> SUCCESS: Tests pass!
3. Read the tests carefully to understand how they cover all use cases.
4- Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>