If the wrong ssh private key is used, and login to the server fails, running the task will also error out

When a user uses wrong private ssh key to login to the server, that fails.
It's bad that the `ssh:doctor` also fails on task `remote:agent_running`.

Any private ssh key can be specified like this:

```
server "111.112.113.114", user: "deploy", roles: %w{web app db}, keys: "/foo/bar.pem"
```

I'm not sure if the key that's specified in this way can have `agent_forwarding` ssh feature..
