How to see actual command errors in Go when shelling out

cmd_output.go

package main

import (
	"bytes"
	"fmt"
	"os/exec"
)

func main() {

	srcFile := "file.mp3"
	destFolder := "D:\\mp3s\\"
	cpCmd := exec.Command("cmd.exe", "/c", "copy", srcFile, destFolder)

	var out bytes.Buffer
	var stderr bytes.Buffer
	cpCmd.Stdout = &out
	cpCmd.Stderr = &stderr

	err := cpCmd.Run()

	if err != nil {
		fmt.Println(fmt.Sprint(err) + ": " + stderr.String())
		fmt.Println("Result: " + out.String())
		return
	}
	fmt.Println("Result: " + out.String())
}