is there a way to know whether ack/nack succeeds or fails

From the source code, no callback or returned error is found.

```go
type Consumer interface {
// Ack the consumption of a single message
Ack(Message)

// Acknowledge the failure to process a single message.
//
// When a message is "negatively acked" it will be marked for redelivery after
// some fixed delay. The delay is configurable when constructing the consumer
// with ConsumerOptions.NAckRedeliveryDelay .
//
// This call is not blocking.
Nack(Message)

// ...
}

```