fixed unhandled exception

lib/single-file/single-file-browser.js

@@ -76,7 +76,7 @@ this.SingleFile = this.SingleFile || (() => {
 				}
 			}
 			if (resourceContent.status >= 400) {
-				resourceContent = options && options.asDataURI ? "data:base64," : "";
+				return options && options.asDataURI ? "data:base64," : "";
 			}
 			let contentType = resourceContent.headers && resourceContent.headers.get("content-type");
 			let charSet;