Do not throw an Error embedding another Error

Should fix #68, but doesn't come with an automated test

lib/cartodb/carto_data.js

@@ -106,7 +106,7 @@ module.exports = function() {
                 if (check_result === 1) {
                     // authorized by key, login as db owner 
                     that.getId(req, function(err, user_id) {
-                        if (err) throw new Error(err);
+                        if (err) throw err;
                         var dbuser = _.template(global.settings.postgres_auth_user, {user_id: user_id});
                         _.extend(req, {dbuser:dbuser});
                         callback(err, true);