I'd like to argue that to a great degree, we can already do error handling the way the proposal demonstrates it in the 'corrected' new example through the tools we already have without introducing new keywords or new magic.
The following achieves the same error handling in almost exactly the same way without being much longer or uglier than the reference material.
func CopyFile(src, dst string) (err error) {
defer func() {
if err != nil {
err = fmt.Errorf("copy %s %s: %v", src, dst, err)
}