Check redis connection at pool creation time

lib/cartodb/redis_pool.js

@@ -58,8 +58,11 @@ var RedisPool = function(opts){
           client.send_anyway = true;
           client.select(database);  
           client.send_anyway = false;
+          callback(null, client);
         });    
-        return callback(null, client);
+        client.on('error', function (err) {
+          callback(err, null);
+        });
       },
       destroy: function(client) { 
         return client.quit();