code: func (e *ErrorGroup) Add(err error) {
  	if err == nil {
  		return
  	}
  	if e.limit > 0 && len(e.Errors) == e.limit {
  		e.excess++
  	} else {
  		e.Errors = append(e.Errors, err)
  	}
  }
docs: // Add is called with errors. nil errors are ignored.