Add functionName to invalid function error (#2963)

Currently in the logs if an invalid function is called the error
message just says "invalid function" which is decidedly unhelpful
when looking through server logs.  Now it will say
"Invalid function: "functionName""

src/Routers/FunctionsRouter.js

@@ -161,7 +161,7 @@ export class FunctionsRouter extends PromiseRouter {
         theFunction(request, response);
       });
     } else {
-      throw new Parse.Error(Parse.Error.SCRIPT_FAILED, 'Invalid function.');
+      throw new Parse.Error(Parse.Error.SCRIPT_FAILED, `Invalid function: "${functionName}"`);
     }
   }
 }