GCS_MAVLink: return MAV_RESULT_FAILED from do_aux_function if invalid function

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -2473,7 +2473,11 @@ MAV_RESULT GCS_MAVLINK::handle_command_do_aux_function(const mavlink_command_lon
     }
     const RC_Channel::AUX_FUNC aux_func = (RC_Channel::AUX_FUNC)packet.param1;
     const RC_Channel::AuxSwitchPos position = (RC_Channel::AuxSwitchPos)packet.param2;
-    rc().do_aux_function(aux_func, position);
+    if (!rc().do_aux_function(aux_func, position)) {
+        // note that this is not quite right; we could be more nuanced
+        // about our return code here.
+        return MAV_RESULT_FAILED;
+    }
     return MAV_RESULT_ACCEPTED;
 }