Make notifier API resilient to bad enrollments

Previously, the user endpoint would would return 404 for any request
involving a user having an enrollment for a course that no longer
existed.
fixup! Make notifier API resilient to bad enrollments

Add explicit course key to enrollment for non-existent course in test case
Merge pull request #5489 from edx/gprice/notifier-api-bogus-enrollment

Make notifier API resilient to bad enrollments