Show a message if player attempts to access postprocessor hud when postprocessing is disabled

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -2079,7 +2079,10 @@ namespace MWGui
     void WindowManager::togglePostProcessorHud()
     {
         if (!MWBase::Environment::get().getWorld()->getPostProcessor()->isEnabled())
+        {
+            messageBox("Postprocessor is not enabled.");
             return;
+        }
 
         bool visible = mPostProcessorHud->isVisible();