Bugfix: Improve error handling for `rclone` backend

Since restic 0.16.0, if rclone encountered an error while listing files,
this could in rare circumstances cause restic to assume that there are no
files. Although unlikely, this situation could result in data loss if it
were to happen right when the `prune` command is listing existing snapshots.

Error handling has now been improved to detect and work around this case.

https://github.com/restic/restic/issues/4612
https://github.com/restic/restic/pull/4618
