Enhancement: Improve retry handling for flaky `rclone` backends

Since restic 0.17.0, the backend retry mechanisms rely on backends correctly
reporting when a file does not exist. This is not always the case for some
`rclone` backends, which caused 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
