오류와 재시도

_, err := client.Emails.Send(ctx, openemail.Body{	"from":    "[email protected]",	"to":      "[email protected]",	"subject": "Your September invoice",	"text":    "Invoice attached.",}) var failure *openemail.Error switch {case err == nil:	fmt.Println("sent")case errors.Is(err, openemail.ErrValidation):	errors.As(err, &failure)	fmt.Println(failure.Code, failure.Param, failure.Message)case errors.Is(err, openemail.ErrRateLimited):	errors.As(err, &failure)	fmt.Println("try again in", failure.RetryAfter)case errors.Is(err, openemail.ErrTimeout):	fmt.Println("no answer in time")default:	return err}