Copy integrity changes

[tuncbkose]

• b02d45b: Adds error messages when fetch/fetch_feedback fails
• 3799451: Wraps copying function into try/except/finally to ensure that in the event of an error, already copied files have correct permissions
• 7fdbae2: Wraps copying function into try/except to ensure that even if not all files were copied, timestamp.txt is still created and feedback workflow doesn't break later
  • Right now, the student receives a "Submission Error" and no indication that something is submitted. However, files are still copied to the exchange folder.
  • I think it is fine to not prevent any copying (i.e. delete already-copied files if an error is encountered at any point), because if a student sees "Submission Error", they will likely try submitting again, hopefully after the problem is fixed.

[github-actions]

👈 Launch a Binder on branch AaltoSciComp/nbgrader/copy-integrity