Minor API changes

Just wondering if it would be better to do it that way. If `reponse.value` is `None`, you will throw a non-documented exception. Maybe I'm wrong, but as a interface user, when a Future is returned, I assume the Future will fail if there's an exception in the method.
```
response.value match {
    case Some(f) if response.code < 300 => f
    case _ => Future.failed(throw new ScnClientException("SCN failure ${response.code}"))
}
```

It is not a suggestion necessary, just want to open the discussion