add error msg for unimplemented methods

engine/src/shell/platform/unity/darwin/macos/uiwidgets_panel.mm

 
 int UIWidgetsPanel::RegisterTexture(void* native_texture_ptr) {
   //TODO: add implementation
+  std::cerr << "registering external texture is not implemented for MacOS" << std::endl;
+  std::cerr << "unregistering external texture is not implemented for MacOS" << std::endl;
 }
 
 std::chrono::nanoseconds UIWidgetsPanel::ProcessMessages() {