code: func (l *Log) Errorf(format string, args ...interface{}) {
  	opts := getLoggerOpts(l.module, api.ERROR)
  	if !opts.levelEnabled {
  		return
  	}
  	if l.loadCustomLogger() {
  		l.customLogger.Errorf(format, args...)
  		return
  	}
  	l.logf(opts, api.ERROR, format, args...)
  }
docs: // Errorf calls go log.Output.
  // Arguments are handled in the manner of fmt.Printf.