print 'Not found' on invalid path

lib/server.js

@@ -32,7 +32,11 @@ function asset_request(req, callback) {
         });
       });
     } else {
-      callback({});
+      callback({
+        body: "Not found",
+        status: -2,
+        code: 404,
+      });
     }
   });
 }