Copter: Notify arming failure when using MAVLink

ArduCopter/GCS_Mavlink.pde

@@ -1273,6 +1273,7 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
                     init_arm_motors();
                     result = MAV_RESULT_ACCEPTED;
                 }else{
+                    AP_Notify::flags.arming_failed = true;  // init_arm_motors function will reset flag back to false
                     result = MAV_RESULT_UNSUPPORTED;
                 }
             } else if (packet.param1 == 0.0f)  {