changed error message for single threaded spinner (#1164)

* changed message to fatal stream and std::runtime_error message to be the same and not imply that spinning was successful when mixing single threaded spinner with multi/async spinners

* updated error messages to reflect behavior when calling a singlethreaded spinner alongside a multithreaded one

* changed position of whitespace

clients/roscpp/src/libros/spinner.cpp

@@ -113,8 +113,7 @@ struct SpinnerMonitor
 
 SpinnerMonitor spinner_monitor;
 const std::string DEFAULT_ERROR_MESSAGE =
-    "Attempt to spin a callback queue from two spinners, one of them being single-threaded. "
-    "This will probably result in callbacks being executed out-of-order.";
+    "Attempt to spin a callback queue from two spinners, one of them being single-threaded.";
 }
 
 namespace ros
@@ -130,8 +129,9 @@ void SingleThreadedSpinner::spin(CallbackQueue* queue)
 
   if (!spinner_monitor.add(queue, true))
   {
-    ROS_FATAL_STREAM("SingleThreadedSpinner: " << DEFAULT_ERROR_MESSAGE);
-    throw std::runtime_error("There is already another spinner on this queue");
+    std::string errorMessage = "SingleThreadedSpinner: " + DEFAULT_ERROR_MESSAGE + " You might want to use a MultiThreadedSpinner instead.";
+    ROS_FATAL_STREAM(errorMessage);
+    throw std::runtime_error(errorMessage);
   }
 
   ros::WallDuration timeout(0.1f);
@@ -220,8 +220,9 @@ void AsyncSpinnerImpl::start()
 
   if (!spinner_monitor.add(callback_queue_, false))
   {
-    ROS_FATAL_STREAM("AsyncSpinnerImpl: " << DEFAULT_ERROR_MESSAGE);
-    throw std::runtime_error("There is already a single-threaded spinner on this queue");
+    std::string errorMessage = "AsyncSpinnerImpl: " + DEFAULT_ERROR_MESSAGE;
+    ROS_FATAL_STREAM(errorMessage);
+    throw std::runtime_error(errorMessage);
   }
 
   continue_ = true;