return 404 in auditor apis if not configured

test-plan:
 - de-configure the auditors cassandra db (or don't have it configured
   in the first place)
 - attempt to access the authentication audit log API
 - should get a 404 Not Found response, not a 500 Internal Server Error
 - repeat for the grade change audit log API

Change-Id: I176d9bbf1265f372c2efe9933a8b97c4cbe9b363
Reviewed-on: https://gerrit.instructure.com/28288
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>