Provide custom onError handler for http-proxy-middleware (facebook#502)

* Change http-proxy-middleware logLevel from silent to error

* provide onError handler for httpProxyMiddleware

scripts/start.js

@@ -170,6 +170,23 @@ function openBrowser(port, protocol) {
   opn(protocol + '://localhost:' + port + '/');
 }
 
+// We need to provide a custom onError function for httpProxyMiddleware.
+// It allows us to log custom error messages on the console.
+function onProxyError(proxy) {
+  return function(err, req, res){
+    var host = req.headers && req.headers.host;
+    console.log(
+      chalk.red('Proxy error:') + ' Could not proxy request ' + chalk.cyan(req.url) +
+      ' from ' + chalk.cyan(host) + ' to ' + chalk.cyan(proxy) + '.'
+    );
+    console.log(
+      'See https://nodejs.org/api/errors.html#errors_common_system_errors for more information (' + 
+      chalk.cyan(err.code) + ').'
+    );
+    console.log();
+  }
+}
+
 function addMiddleware(devServer) {
   // `proxy` lets you to specify a fallback server during development.
   // Every unrecognized request will be forwarded to it.
@@ -209,6 +226,7 @@ function addMiddleware(devServer) {
       httpProxyMiddleware(pathname => mayProxy.test(pathname), {
         target: proxy,
         logLevel: 'silent',
+        onError: onProxyError(proxy),
         secure: false,
         changeOrigin: true
       })