Using a different SSHKit runner doesn't work well, because the group
runner uses the Parallel runner internally. So instead we'll patch its
behaviour to fail slow.
We'll also get it to return all the errors so we can report on all the
hosts that failed.
The cause message doesn't include the host the error occurred on.
Before:
```
$ mrsk deploy
Acquiring the deploy lock...
Finished all in 0.1 seconds
ERROR (SocketError): getaddrinfo: nodename nor servname provided, or not known
```
After:
```
$ mrsk deploy -d staging
Acquiring the deploy lock...
Finished all in 0.1 seconds
ERROR (SocketError): Exception while executing on host server-123: getaddrinfo: nodename nor servname provided, or not known
```
