[Gordon] added the error handling example

Author: songguoqiang

error_handler_example.js

@@ -0,0 +1,21 @@
+const express = require("express");
+const app = express();
+
+app.get("/", function(req, res, next) {
+  // assume some error happens
+  const err = new Error("Unexpected network error");
+  next(err);
+});
+
+app.get("/", function(req, res, next) {
+  console.log("You should not see this line in the console.");
+});
+
+app.use(function(err, req, res, next) {
+  res.status(500);
+  res.send({ error: "unknown error" });
+});
+
+const server = app.listen(3000, function() {
+  console.log("Application started....");
+});