Bug 28020: Add error_code to error responses

Bug 28020: Add error_code to error responses

This patch adds the *error_code* attribute to the error response
definition. This way it can be added when returning an error.

I also made a sample usage of it on the generic unhandled_exception
helper, which the unit tests on this patchset expect.

To test:
1. Apply the unit tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/unhandled_exceptions.t
=> FAIL: The unhandled_exception() helper doesn't return an error_code
in the response payload.
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>