Snapper setup fails if /.snapshots subvolume already exists

Feedback on a Problem or for Improvement

Summary
snapper configuration fails if it was previously installed

Description
If snapper was previously installed, and a btrfs /.snapshots subvolume already exists, that should be recognized, and the subvolume should be (re)used. Instead setup of Freedombox silently fails, unless one watches the details as they unfold by starting plinth from the command line (and then deleting things in a different terminal).

INFO plinth.operation     Operation: snapshot: Installing app: added
INFO plinth.operation     Operation: snapshot: Installing app: running
INFO plinth.setup         Setup run: snapshot
INFO plinth.actions       # snapshot..setup(…) 
ERROR plinth.actions       Error running action snapshot..setup(*(0,), **{}): Command '['snapper', 'create-config', '/']' returned non-zero exit status 1. (1, ['snapper', 'create-config', '/'], b'', b'Creating config failed (creating btrfs subvolume .snapshots failed since it already exists).\n\x1b[31m   ERROR\x1b[0m \x1b[94m__main__            \x1b[0m Error executing action: Command \'[\'snapper\', \'create-config\', \'/\']\' returned non-zero exit status 1.\nTraceback (most recent call last):\n  File "/usr/share/plinth/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n  File "/usr/lib/python3/dist-packages/plinth/modules/snapshot/privileged.py", line 29, in setup\n    subprocess.run(command, check=True)\n  File "/usr/lib/python3.10/subprocess.py", line 526, in run\n    raise CalledProcessError(retcode, process.args,\nsubprocess.CalledProcessError: Command \'[\'snapper\', \'create-config\', \'/\']\' returned non-zero exit status 1.\n') ['  File "/usr/share/plinth/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n', '  File "/usr/lib/python3/dist-packages/plinth/modules/snapshot/privileged.py", line 29, in setup\n    subprocess.run(command, check=True)\n', '  File "/usr/lib/python3.10/subprocess.py", line 526, in run\n    raise CalledProcessError(retcode, process.args,\n']
INFO plinth.setup         Setup completed: snapshot: Error installing app: Command '['snapper', 'create-config', '/']' returned non-zero exit status 1. Command '['snapper', 'create-config', '/']' returned non-zero exit status 1.
ERROR plinth.setup         Error running setup - Command '['snapper', 'create-config', '/']' returned non-zero exit status 1.
WARNING plinth.setup         Unable to complete setup: Command '['snapper', 'create-config', '/']' returned non-zero exit status 1.
INFO plinth.setup         Will try again in 10 seconds

Proposed Solution
Setup should just continue and reuse the existing subvolue