errors.go

package main

import "fmt"

type customErr struct {
        Value string
}

func (c *customErr) Error() string {
        return c.Value
}

func DoSomething(fail bool) *customErr {
        if fail {
                return &customErr{Value: "failed"}
        }
        return nil
}

func main() {
        var err error
        err = DoSomething(true)
        if err != nil {
                fmt.Println(err)
        }
        err = DoSomething(false)
        if err != nil {
                fmt.Println("also failed")
        }
}