added error handling on http-proxy instance

lib/modules/http.js

@@ -204,6 +204,13 @@ module.exports.create = function (deps, conf, greenlockMiddleware) {
     , toProxy: true
     });
 
+    proxy.on('error', function (err, req, res) {
+      res.statusCode = 502;
+      res.setHeader('Connection', 'close');
+      res.setHeader('Content-Type', 'text/html');
+      res.end(require('../proxy-conn').getRespBody(err, conf.debug));
+    });
+
     proxyServer = http.createServer(function (req, res) {
       proxy.web(req, res, req.connection.proxyOpts);
     });