Enhancement: Improve handling of flaky rclone backends

Since restic 0.17.0, the backend retry mechanisms relies on backends correctly
reporting when a file does not exist. This is not always the case for some rclone
backends, causing restic to stop retrying after the first failure.

For rclone, failed requests are now retried up to 5 times before giving up.

https://github.com/restic/restic/pull/5251
