var (
	ErrRegisterPet  errific.Err = "error registering pet"
	ErrValidateKind errific.Err = "only cats are allowed, cannot register '%s'"
)

func main() {
	if err := registerPet("hamster"); err != nil {
		switch {
		case errors.Is(err, ErrValidateKind): // 400 errors
			fmt.Println(http.StatusBadRequest, err)

		default: // 500 errors
			fmt.Println(http.StatusInternalServerError, err)
		}
	}
}

func registerPet(kind string) error {
	if err := validateKind(kind); err != nil {
		return ErrRegisterPet.New(err)
	}
	return nil
}

func validateKind(kind string) error {
	if kind != "cat" {
		return ErrValidateKind.Errorf(kind)
	}
	return nil
}