Raise error for incomplete response without new server state

[CrowdHailer]

If any callback returns a response with body set to true an error should be raised. This is because the response is incomplete.

e.g.

defmodule MyServer do
use Raxx.Server

def handle_request(request, state) do
  response = Raxx.response(:ok)
  |> Raxx.set_body(true)
end
end

This example should raise an error the correct return value is {response, new_state}