goerror.go

package main

import (
	"fmt"
)

type Err struct {
	m string
}

func (e *Err) Error() string {
	return e.m
}

func getError() *Err {
	return nil
}

func main() {
	err := fmt.Errorf("test")

	err = getError()
	
	if err != nil {
		fmt.Printf("got error")
		return
	}
	fmt.Printf("no error")
}

possible_solution.go

package main

import (
	"fmt"
)
type CustomError interface {
	additionalInfo() string
	error
}

type Err struct {
	m string
}

func (e *Err) additionalInfo() string {
	return "additional info"
}

func (e *Err) Error() string {
	return e.m
}

func getError() CustomError {
	return nil
}

func main() {
	err := fmt.Errorf("test")

	err = getError()
	
	if err != nil {
		fmt.Printf("got error")
		return
	}
	fmt.Printf("no error")
}