Bugfix: Do not retry load/list operation is 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 is broken.

This has been fixed now.

https://github.com/restic/restic/pull/5101
https://forum.restic.net/t/restic-hanging-on-backup/8559/2