add message when failed to load thumbs from cloud

engine/app_cloud.cpp

@@ -112,18 +112,31 @@ void App::cloud_browse()
 
     dialog->btn_ok->on_click = [this, dialog](Node*)
     {
+        dialog->destroy();
         std::thread([this, dialog] {
+            async_start();
+            auto* m = layout[main_id]->add_child<NodeMessageBox>();
+            m->m_title->set_text("Downloading");
+            m->m_message->set_text("Download in progress, please wait...");
+            async_update();
+            async_end();
+            
             download(dialog->selected_file);
+            
             async_start();
             canvas->reset_camera();
             layers->clear();
+            async_end();
+            
             canvas->m_canvas->project_open_thread(dialog->selected_path);
+            
+            async_start();
             doc_name = dialog->selected_name;
             title_update(doc_name, canvas->m_canvas->m_width);
             for (auto& i : canvas->m_canvas->m_order)
                 layers->add_layer(canvas->m_canvas->m_layers[i].m_name.c_str());
-            dialog->destroy();
             ActionManager::clear();
+            m->destroy();
             async_update();
             async_end();
         }).detach();