A CopyFile function in Go with correct error handling

CopyFile-correct.go

func CopyFile(dstName, srcName string) (int64, error) {
	src, e := os.Open(srcName)
	if e != nil {
		return 0, errors.New("Error while opening file for reading. Caused by: " + e.Error())
	}

	dst, e := os.Create(dstName)
	if e != nil {
		src.Close()
		return 0, errors.New("Error while opening file for writing. Caused by: " + e.Error())
	}

	numBytesWritten, e := io.Copy(dst, src)
	if e != nil {
		dst.Close()
		src.Close()
		return 0, errors.New("Error while copying. Caused by: " + e.Error())
	}

	e = dst.Close()
	if e != nil {
		src.Close()
		return numBytesWritten, errors.New("Error while closing. Caused by: " + e.Error())
	}

	e = src.Close()
	if e != nil {
		return numBytesWritten, errors.New("Error while closing. Caused by: " + e.Error())
	}

	return numBytesWritten, nil
}