Error message when server disconnected

If the server isn't running and we `raco ipoe db` anyway, this happens:

```
tcp-connect: connection failed
  address: localhost
  port number: 5432
  step: 6
  system error: Connection refused; errno=111
```

Print a nicer message instead.

Also, allow customizing the default user in `ipoe init`. So it's not always postgres.
