Bug 28020: Unit tests

Bug 28020: Unit tests

The unhandled_exception() Mojo helper didn't have proper tests. As this
bug is adding a slight behavior change, I tried to add some. As it
relies on the OpenAPI plugin, it cannot be done the usual way using
Mojo::Lite. So I picked a route and forced an exception through mocking
to be able to write tests.

To test:
1. Apply this 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.

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>