Bugfix: Do not retry load/list operation if SFTP connection is broken

When using restic with the SFTP backend, backend operations that load a
file or list files were retried even if the SFTP connection was broken.
This has now been fixed.

https://github.com/restic/restic/pull/5101
https://forum.restic.net/t/restic-hanging-on-backup/8559
