log.go
func (l *slog) Errorf(format string, args ...interface{}) {
	lvl := l.Level()
	if lvl <= LevelError {
		l.b.printf("ERR", l.tag, format, args...)
	}
}
func (l *slog) Errorf(format string, args ...interface{}) {
	lvl := l.Level()
	if lvl <= LevelError {
		l.b.printf("ERR", l.tag, format, args...)
	}
}
// Errorf formats message according to format specifier, prepends the prefix as
// necessary, and writes to log with LevelError.
//
// This is part of the Logger interface implementation.