We've had quite a few instances recently (eg bug 1270167, bug 1269629) where people have had failures submitting to the API and the resultant HTTP 500 gave them no clue as to what was wrong with the payload. We should allow people to fix these issues without requiring us from having to dig in the logs :-) There's also bug 1266990 for which there isn't yet a corresponding treeherder-improve-UX bug.