Emit middlewareError when on middleware error.

Now it's possible to pass an Error object to next() and have it handled
in a custom way that's suitable to your application.

lib/node-http-proxy.js

@@ -294,6 +294,10 @@ exports.stack = function stack (middlewares, proxy) {
     handle = function (req, res) {
       var next = function (err) {
         if (err) {
+          if (! proxy.emit('middlewareError', err, res, req)) {
+            console.error('Error in middleware(s): %s', err.stack);
+          }
+
           if (res._headerSent) {
             res.destroy();
           }
@@ -303,7 +307,6 @@ exports.stack = function stack (middlewares, proxy) {
             res.end('Internal Server Error');
           }
 
-          console.error('Error in middleware(s): %s', err.stack);
           return;
         }