fix can't tap retry button on mobile devices

Author: krmbn0576

js/rpg_core/Graphics.js

@@ -288,6 +288,10 @@ Graphics.printLoadingError = function(url) {
         button.style.color = '#ffffff';
         button.style.backgroundColor = '#000000';
         button.onclick = ResourceHandler.retry.bind(ResourceHandler);
+        button.ontouchstart = function(event) {
+            ResourceHandler.retry();
+            event.stopPropagation();
+        };
         this._errorPrinter.appendChild(button);
         this._loadingCount = -Infinity;
     }