Don't return error output when the cartfs user does not exists.

Make sure the cartfs user exists as findmnt will return mounts started by root if searching with an empty user id.

Remove the empty echo as it does not seem to be needed.

Send log from cartfs workspace deletion into a log file instead to stdout.

Bug: 430136204
Tested: tested locally with and without the cartfs user.
Change-Id: I7782e1bb23b5f527cc7f45b85b18a352516dec00