[minor] More contextual errors when middleware(s) error

lib/node-http-proxy.js

@@ -125,16 +125,19 @@ exports.stack = function stack (middlewares, proxy) {
     handle = function (req, res) {
       var next = function (err) {
         if (err) {
-          console.error(err.stack);
           if (res._headerSent) {
             res.destroy();
-          } else {
+          } 
+          else {
             res.statusCode = 500;
             res.setHeader('Content-Type', 'text/plain');
             res.end('Internal Server Error');
           }
+          
+          console.error("error in middleware layer: %s", err.stack);
           return;
         }
+        
         child(req, res);
       }