usage.go
func (p *Parser) Fail(msg string) {
	p.WriteUsage(os.Stderr)
	fmt.Fprintln(os.Stderr, "error:", msg)
	os.Exit(-1)
}
func (p *Parser) Fail(msg string) {
	p.WriteUsage(os.Stderr)
	fmt.Fprintln(os.Stderr, "error:", msg)
	os.Exit(-1)
}
// Fail prints usage information to stderr and exits with non-zero status