Bug 26515: (QA follow-up) Preserve original behaviour

Bug 26515: (QA follow-up) Preserve original behaviour

This patch removes the use of $self->items->safe_delete, as we don't
want to change the current behaviour (i.e. delete what can be deleted)
As safe_delete would rollback entirely, there was a behaviour change.

Now items are deleted in a loop that catches any problem and reports it
using the new ->add_message mechanism. The $item object is added to the
message payload so it doesn't need to be queried by the caller for
providing UI feedback.

Tests are augmented accordingly,

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

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>