AFNetworking3.x获得failure时候的respo

通过失败block内的参数error取得，代码如下：

NSData *data = (NSData *)error.userInfo[AFNetworkingOperationFailingURLResponseDataErrorKey] ;
NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:nil];