The way code usually looks like is this:

  func foo(arg int) error {
    if arg < 0 {
      return fmt.Errorf("expected >0, got %d", arg)
    }
    if arg > 8 {
      return fmt.Errorf("expected >8, got %d", arg)
    }
    //do stuff
  }

  func foo(arg int) error {
    if arg < 0 {
      return errors.New(fmt.Sprintf("expected >0, got %d", arg))
    }
    if arg > 8 {
      return errors.New(fmt.Sprintf("expected >8, got %d", arg))
    }
    //do stuff
  }