glg.go
func (g *Glg) FailFunc(f func() string) error {
	if g.isModeEnable(FAIL) {
		return g.out(FAIL, "%s", f())
	}
	return nil
}
func (g *Glg) FailFunc(f func() string) error {
	if g.isModeEnable(FAIL) {
		return g.out(FAIL, "%s", f())
	}
	return nil
}
// FailFunc outputs Fail level log returned from the function