Member unregistration errors when member already unregistered.

```
$ fly machines remove 5683256c73258e
machine 5683256c73258e was found and is currently in stopped state, attempting to destroy...
unregistering postgres member 'fdaa:0:2e26:a7b:7d17:4463:955d:2' from the cluster... (failed)
failed to unregister postgres member: failed to resolve member: no rows in result set
(success)
5683256c73258e has been destroyed
```

If the member has already been unregistered, then we don't need to return an error. Also, there's clearly an issue with the output if (success) is indicated along with the failure.