Do not abort backup if can't fetch one of the remote target

As title, I configured a multi remote target backup but sometimes one of those remote nodes is not reachable, there's a way to skip those node's backup and do the rest? Because right now even if one is not reachable the backup fails for all of the remote target

Sample configuration
```
volume
[...]
subvolume
target send-receive
ssh_identity [...]
target send-receive
ssh_identity [...]
target send-receive
ssh_identity [...]
```

Start by tracing the multi-target `send-receive` backup flow described in the sample configuration, focusing on how an unreachable target affects the other targets. Define the completed behavior as skipping the failed remote target, continuing backups for reachable targets, and reporting the failure clearly.

Written by the indexing model from the issue text.