Handle out-of-band file move/deletion

[dlqqq]

Description

jupyter-server-documents should handle file deletion gracefully. IMO, we should use a reserved close code to indicate when a file was deleted, and show a dialog to the user whenever this happens.

We should also:

• Verify notebooks can be opened after another was deleted: Handle YDoc reset & out-of-band changes in notebooks #93 (comment)

• Add logging for when we cannot resolve the file ID to a file path: Handle YDoc reset & out-of-band changes in notebooks #93 (comment)