feat(default_ad_api): add log when operation mode change fails

Signed-off-by: Takagi, Isamu <[email protected]>

system/default_ad_api/src/operation_mode.cpp

@@ -66,8 +66,13 @@ void OperationModeNode::change_mode(
   const ResponseT res, const OperationModeRequest::_mode_type mode)
 {
   if (!mode_available_[mode]) {
+    RCLCPP_WARN(
+      get_logger(),
+      "The target mode is not available. Please check the cause with "
+      "rqt_diagnostics_graph_monitor");
     throw component_interface_utils::ServiceException(
-      ServiceResponse::ERROR_NOT_AVAILABLE, "The mode change is blocked by the system.");
+      ServiceResponse::ERROR_NOT_AVAILABLE,
+      "The target mode is not available. Please check the diagnostics.");
   }
   const auto req = std::make_shared<OperationModeRequest>();
   req->mode = mode;