GCS_MAVLink: panic in SITL if unknown message sent

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -2324,6 +2324,9 @@ bool GCS_MAVLINK::try_send_message(const enum ap_message id)
         // This message will be sent out at the same rate as the
         // unknown message, so should be safe.
         gcs().send_text(MAV_SEVERITY_DEBUG, "Sending unknown message (%u)", id);
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        AP_HAL::panic("Sending unknown ap_message %u", id);
+#endif
         ret = true;
         break;
     }