the response might have already been sent

fixes https://github.com/c9/newclient/issues/10438

node_modules/vfs-http-adapter/restful.js

@@ -11,6 +11,9 @@ module.exports = function setup(mount, vfs, mountOptions) {
 
     var errorHandler = mountOptions.errorHandler || function (req, res, err, code) {
         console.error(err.stack || err);
+        if (res.headersSent)
+            return;
+            
         if (code) res.statusCode = code;
         else if (typeof err.code == "number") res.statusCode = err.code;
         else if (err.code === "EBADREQUEST") res.statusCode = 400;