Return error message along with a type. https://play.golang.org/p/9QsCFPXv_vV

custom_error.go

package main

import (
	"errors"
	"fmt"
	"strings"
)

// Simple errors
type NotFoundError struct{ s string }

func (e *NotFoundError) Error() string { return e.s }

type BadRequestError struct{ s string }

func (e *BadRequestError) Error() string { return e.s }

// Error as slice
type ConflictError struct{ errors []string }

func (e *ConflictError) Error() string    { return strings.Join(e.errors, ", ") }
func (e *ConflictError) Errors() []string { return e.errors }

// A function returning different error types
func get(id string) error {
	if id == "" {
		return &BadRequestError{"id not given"}
	} else if id == "3" {
		return &NotFoundError{id + " is not found"}
	} else if id == ":-)" {
		return &ConflictError{[]string{"Char : is not allowed", "Char - is not allowed", "Char ) is not allowed"}}
	} else {
		return fmt.Errorf("function not implemented")
	}
}

// Catching different error types
func main() {

	// using type assertion
	err := get("")
	if err != nil {
		if _, ok := err.(*NotFoundError); ok {
			fmt.Printf("not found: %s\n", err)
		} else {
			fmt.Printf("some other error: %s\n", err)
		}
	}

	// using type switch
	err = get("3")
	if err != nil {
		switch e := err.(type) {
		case *NotFoundError:
			fmt.Printf("not found: %s\n", e)
		case *BadRequestError:
			fmt.Printf("bad request: %s\n", e)
		case *ConflictError:
			fmt.Printf("conflict: %s\n", e)
		default:
			fmt.Printf("some other error: %s\n", err)
		}
	}

	// using error.As (Go >= 1.13)
	err = get(":-)")
	var e *ConflictError
	if errors.As(err, &e) {
		fmt.Printf("conflict (string): %s\n", e)
		fmt.Printf("conflict (slice): %s\n", e.Errors())
	}

}