Do not try to send commands to an unoconnected redis client

This changes "Cannot read property 'HGET' of null" messages into
"Redis connection to 127.0.0.1:6379 failed - connect ECONNREFUSED".

lib/cartodb/carto_data.js

@@ -223,6 +223,7 @@ module.exports = function() {
                 redis_pool.acquire(db, this);
             },
             function executeQuery(err, data) {
+                if ( err ) throw err;
                 redisClient = data;
                 redisArgs.push(this);
                 redisClient[redisFunc.toUpperCase()].apply(redisClient, redisArgs);