Wie Golang Fehler definiert

type error interface {
    Error() string
}

package errors

type Error struct {
    ErrCode int
    ErrMsg  string
}

func NewError(code int, msg string) *Error {
    return &Error{ErrCode: code, ErrMsg: msg}
}

func (err *Error) Error() string {
    return err.ErrMsg
}

package main

import (
    "encoding/json"
    "errors"
    "fmt"

    myerr "github.com/mypractise/errors"
)

func myErr() error {

    err := myerr.NewError(2, "err test")
    return err
}

func staErr() error {
    m := make(map[string]string)
    err := json.Unmarshal([]byte(""), m)
    if err != nil {
        return err
    }

    return errors.New("aaaaa")
}

func main() {
    err1 := staErr()
    fmt.Println("------sta err:", err1.Error())

    err2 := myErr()
    fmt.Println("------my err:", err2.Error(), err2.(*myerr.Error).ErrCode)
}

go run ./test.go