mss: Fix bus warning handling

It was calling the error parser which would assert and abort.

src/main.c

@@ -47,8 +47,9 @@ gst_bus_cb (GstBus * bus, GstMessage * message, gpointer data)
     {
       GError *gerr;
       gchar *debug_msg;
-      gst_message_parse_error (message, &gerr, &debug_msg);
-      g_warning ("Error: %s (%s)", gerr->message, debug_msg);
+      gst_message_parse_warning (message, &gerr, &debug_msg);
+      GST_DEBUG_BIN_TO_DOT_FILE (pipeline, GST_DEBUG_GRAPH_SHOW_ALL, "mss-pipeline-WARNING");
+      g_warning ("Warning: %s (%s)", gerr->message, debug_msg);
       g_error_free (gerr);
       g_free (debug_msg);
     }