Use ES6 async/await in the example

Hi, I'm a beginner developer. I think now it's more clear with ES6 Async/await and arrow function.

I didn't use try-catch because throw. I'm just practising! Any suggestion is very welcome

sections/errorhandling/useonlythebuiltinerror.md

@@ -16,12 +16,11 @@ const myEmitter = new MyEmitter();
 myEmitter.emit('error', new Error('whoops!'));
 
 // 'throwing' an Error from a Promise
-return new Promise(function (resolve, reject) {
-    return DAL.getProduct(productToAdd.id).then((existingProduct) => {
-        if(existingProduct != null)
-            reject(new Error("Why fooling us and trying to add an existing product?"));
-    });
-});
+const addProduct = async (productToAdd) => {
+  const existingProduct = await DAL.getProduct(productToAdd.id);
+  if (existingProduct != null)
+      reject(new Error("Why fooling us and trying to add an existing product?"));
+}
 ```
 
 ### Code example – Anti Pattern