message: [qa] add assert on missing connection
  Improves the error message when you forget to .add_connection
diff: @@ -195,6 +195,7 @@ class SingleNodeConnCB(NodeConnCB):
  # Wrapper for the NodeConn's send_message function
  def send_message(self, message, pushbuf = False):
  + assert self.connection is not None, 'forgot to .add_connection'
  self.connection.send_message(message, pushbuf)
  def send_and_ping(self, message):
  