schnapps cli should exit when given invalid arguments
schnapps cleanup 10 or a different command with invalid arguments should fail with an exit code 1 and an error message. Otherwise, users might think that their command was correct and succeeeded, but only the valid part of it will be executed.
For example users might expect
schnapps cleanup 10 to leave 10 most recent snapshots, the 10 argument will be silently ignored.