Use an `error` callback just like regular Backbone/jQuery deferred objects would.

Is it possible to make the Rpc call use an `error` callback by passing the JSON-RPC error response to it? Like this:

```
user.checkLoggedIn({
    success: function(model response){
        // do something
    },
    error: function(model, exception){
        alert(exception.message);
    },
});
```

Where `user` is an instance of a Backbone model and `checkLoggedIn` is one of the RPC methods.
