When creating exam, ensure (in validation), verify flow ID is valid

At the very least, we should check that the flow ID is a valid identifier, otherwise `/list-available-exams/` will fail with a server error, unable to reverse the URL pattern. As a nicety for users, we could also check that the flow ID resolves within the course.

Start at the exam-creation validation flow and the /list-available-exams/ endpoint, which currently fails when the flow ID cannot be used to reverse its URL. Verify that invalid identifiers are rejected before creation, and check whether the ID resolves within the course as an additional validation condition.

Written by the indexing model from the issue text.