Bug 29772: Make DELETE /patrons/:patron_id return error codes

Bug 29772: Make DELETE /patrons/:patron_id return error codes

This patch makes the controller add the error_code attribute on error
conditions payload.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons.t
=> FAIL: Boo, the API doesn't include the error_code attribute on the
error responses for the DELETE /patrons/:patron_id route
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass! error_code is included in the response!
5. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>