notify_on_stop errors if not moving

notify_on_stop now gives an error if it's
called while the stage is not moving.
This means you can safely issue the command and await a response.
If the stage is moving, it will block until
the stage stops.  If it's not moving, it will return immediately.

src/modules/stage/stage.cpp

@@ -281,7 +281,14 @@ void is_stage_moving(String command)
 
 void activate_notify_on_stop(String command)
 {
-    notify_on_stop = true;
+    if (!stage_moving)
+    {
+        Serial.println("Error: stage is not moving");
+    }
+    else
+    {
+        notify_on_stop = true;
+    }
 }
 
 //TODO: move help strings to program memory