restic prune should refuse to work on repositories without snapshots

[MichaelEischer]

What should restic do differently? Which functionality do you think we should add?

Currently prune will wipe a repository, if there are no snapshots in it. If a problem while listing the snapshots occurs, this could even cause prune to completely break a repository. (see #4612 for an example).

As suggested in https://forum.restic.net/t/prune-suspicious-state-detection/7084 , prune could refuse to process a repository without snapshots in it, unless a special flag is passed to it. So far, I've only encountered repositories without snapshots during testing, but I wonder whether there are some valid use cases for this?

An alternative might be that prune retries listing the snapshots multiple time, if an empty list was returned. That would be able to protect against temporary errors, but a persistent error would still be a problem.