Why isn't the response filled if my service returns false?

edit

I have a service callback:

bool myServiceCallback(service_test::MyService::Request &req,
                       service_test::MyService::Response &res)
{
  ROS_INFO_STREAM("Input string = " << req.input);

  if (req.input.empty())
  {
    res.text_error = "";
    return true;
  }
  else
  {
    res.text_error = "Not empty!";
    return false;
  }
}

It returns true if the request string is empty, false otherwise. It also fills a text_error string to indicate the user what happened in case the service returns false.

The client is like this:

 ros::ServiceClient client = nh.serviceClient<service_test::MyService>("my_service");

  bool return_value;
  service_test::MyService srv;

  srv.request.input = "bla";
  return_value = client.call(srv);
  ROS_INFO_STREAM("Return value = " << (return_value ? "True": "False"));
  ROS_INFO_STREAM("Response text error = " << srv.response.text_error);

The terminal output of the client node is the following:

[ INFO] [1501836030.156100067]: Return value = False
[ INFO] [1501836030.156145499]: Response text error =

It feels like the response is not filled if a service returns false. Why is that? How can I return an error string indicating what was wrong?

Using an action instead might help me solve this issue, but I would like to keep the simplicity of a blocking service call.