Bugfix: Prevent `forget --keep-tags invalid` from deleting all snapshots

Running `forget --keep-tags invalid`, where the tag `invalid` does not
exist in the repository, would remove all snapshots. This is especially
problematic if the tag name contains a typo.

The `forget` command now fails with an error if all snapshots in a snapshot
group would be deleted. This prevents the above example from deleting all
snapshots.

https://github.com/restic/restic/issues/4568
https://github.com/restic/restic/pull/4764
https://forum.restic.net/t/delete-all-snapshots-in-one-command-is-this-feature-intentional/6923/3