Bug 28813: Rename delivery_note to failure_code

Bug 28813: Rename delivery_note to failure_code

We now use the codes from the half implimented error_code in place of
the plain text that was being added to the delivery_note field. As part
of that we rename the field to failure_code to clarify it's intended
content.

Test plan
1/ Confirm t/db_dependent/Letters.t passes

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>