Plane: set heartbeat.system_status to Critical for low-battery failsafe events

ArduPlane/GCS_Mavlink.cpp

@@ -14,7 +14,7 @@ void Plane::send_heartbeat(mavlink_channel_t chan)
     uint8_t system_status;
     uint32_t custom_mode = control_mode;
     
-    if (failsafe.state != FAILSAFE_NONE) {
+    if (failsafe.state != FAILSAFE_NONE || failsafe.low_battery) {
         system_status = MAV_STATE_CRITICAL;
     } else if (plane.crash_state.is_crashed) {
         system_status = MAV_STATE_EMERGENCY;