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;
  }