Method Error

examples/custom-errors/custom-errors.go:21–23

Adding this `Error` method makes `argError` implement the `error` interface.

()

// Adding this `Error` method makes `argError` implement
// the `error` interface.
func (e *argError) Error() string {
	return fmt.Sprintf("%d - %s", e.arg, e.message)
}

func f(arg int) (int, error) {
	if arg == 42 {