[api] Manual merge of #46: add custom `proxyError` event and enable production error handling.

lib/node-http-proxy.js

@@ -370,11 +370,29 @@ HttpProxy.prototype.proxyRequest = function (req, res, options) {
   //
   function proxyError(err) {
     errState = true;
+    
+    //
+    // Emit an `error` event, allowing the application to use custom
+    // error handling. The error handler should end the response.
+    //
+    if (self.emit('proxyError', err, req, res)) {
+      return;
+    }
+
     res.writeHead(500, { 'Content-Type': 'text/plain' });
 
     if (req.method !== 'HEAD') {
+      //
+      // This NODE_ENV=production behavior is mimics Express and
+      // Connect.
+      //
+      if (process.env.NODE_ENV === 'production') {
+        res.write('Internal Server Error');
+      }
+      else {
         res.write('An error has occurred: ' + JSON.stringify(err));
       }
+    }
   
     res.end();
   }