Validate inputs in Review Toolbar

Currently the review tool feedback form does not validate the user token or MR ID before the user attempts to submit the feedback. In gitlab-ce!30481 we've updated the error messaging so that the user is prodded to understand what a 401 or 404 might mean.

Do we want to test these when then users enter them (probably by hitting an appropriate API endpoint)? This would add two server roundtrips, but would result in sooner validation. What would be the best endpoints to use? TDB! Is there a better way to do this? Probably!

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30481#note_189886462