`string' does not contain a definition for `error' and no extension method `error' of type `string' could be found (are you missing a using directive or an assembly reference?)

Hi gang were do i Assign error to so unity knows if their is an error?

 IEnumerator GetUserAccount(int Gold, string name)
         {
         //all acount items, here...
         string hash = MD5.Md5Sum(secretKey);
         string Account_get = GetAccount + "name=" + name + "hash=" + hash;
 
             yield return Account_get;
             
             if (Account_get.error != null)
             {
             print("Their was an error getting Account, Account dose not Exist" + Account_get.error);
             }
             else
             {
                 //DISPLAY EACH STRING ITEM AS SEPRET TEXT....
             gameObject.GetComponent<GUIText>().text = Gold.ToString();
             }
         }

best overloaded method error? i have assigned callback function in the script. to get id data

 void Start()
         {
         FB.API("/me?fields=id", Facebook.HttpMethod.GET, APICallback);
         StartCoroutine(GetUserAccount());
         }