Avoid swallowing rejected font listing promise

src/serve_font.js

@@ -54,9 +54,7 @@ module.exports = function(options, allowedFonts) {
     );
   });
 
-  return new Promise(function(resolve, reject) {
+  return fontListingPromise.then(function() {
-    fontListingPromise.then(function() {
+    return app;
-      resolve(app);
-    });
   });
 };