A way for scripts to signal errors

Description

Scripts can fail in ways that are not the fault of a student. For example, Docker images can disappear from the repository. It should be possible for scripts to send back errors instead of feedback so that submissions are not stuck on "running".

There should be a new endpoint or some mode of using /script/feedback that can tell Submit that the script failed to do its thing.

Checklist

• Add some way for scripts to signal errors
• Optionally, accept a message for diagnosis by staff
• The student is informed that the script failed, but it's likely not their fault