Bug 20624: (QA follow-up) Handle missing deps gracefuly

Bug 20624: (QA follow-up) Handle missing deps gracefuly

This patch makes the /token endpoint and the authenticate_api_request
method behave correctly in the event of missing deps for OAuth2.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/oauth.t
=> FAIL: The behaviour is not implemented
- Apply this patch
- Run:
 k$ prove t/db_dependent/api/v1/oauth.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>