Non-existent kept files cause view crashes

[sb10]

Using keep --view on a directory where files have been kept, can fail:

vault keep --view mine
2022-04-05T14:07:35Z+0100	CRITICAL	[Errno 2] No such file or directory: '/.../.git/objects/23/9de85972c940378e6d122e8be7731378fef4ad'
  File "/.../bin/vault", line 8, in <module>
    sys.exit(main())
  File "/.../vault/.venv/lib/python3.8/site-packages/bin/vault/__init__.py", line 209, in main
    view(Branch.Keep, _view_contexts[context], args.absolute)
  File "/.../vault/.venv/lib/python3.8/site-packages/bin/vault/__init__.py", line 72, in view
    elif view_mode == ViewContext.Mine and path.stat().st_uid != os.getuid():
  File ".../conda_envs/R.4/lib/python3.8/pathlib.py", line 1197, in stat
    return self._accessor.stat(self)

Likely due to the kept file having now been deleted.

[sb10]

As a user, I want to be able to view remaining kept files, even if I delete some of them