message: display error message
diff: @@ -100,7 +100,7 @@ message_server::message_server()
  if (ec != 0)
  {
  std::ostringstream os;
  - os << "listen failed on port " << port << ", " << ec;
  + os << "listen failed on port " << port << ", " << ec.message();
  throw std::runtime_error(os.str());
  }
  else
  