Add test to ensure backup tasks properly handle non-existent snapshots by: - Verifying backup task reports failure status - Ensuring error is propagated through task status API Previously untested edge case when backing up a snapshot that doesn't exist in the test_backup.py tests. Refs scylladb/scylladb#21381 Signed-off-by: Kefu Chai <kefu.chai@scylladb.com> Closes scylladb/scylladb#21385