Cleanup snapshots anyway even if one target aborted

I have multiple external HDDs that I backup to, but I only keep 1 external HDD available near me.

When I backup with `btrbk`, it will refuse to cleanup the snapshots if one of my targets isn't available, even though backing up to 1 HDD was fine.

Can there be a way to tell `btrbk` to cleanup the snapshots anyway?

The issue names btrbk and snapshot cleanup but no source file, test, or entry point. Start by locating the logic that decides whether cleanup runs after a target abort and the configuration or CLI handling around it; done means an explicit way to allow cleanup after an unavailable target while preserving the successful backup behavior.

Written by the indexing model from the issue text.