code: func (b *bundle) executeErrorTemplate(templateName string, input *EmailTemplateInput, err error) (subject, body string) {
  	subject = fmt.Sprintf(`[Build Status] Builder %q`, protoutil.FormatBuilderID(input.Build.Builder))
  
  	errorTemplateInput := map[string]interface{}{
  		"Build":        input.Build,
  		"TemplateName": templateName,
  		"TemplateURL":  b.defURLs[templateName],
  		"Error":        err.Error(),
  	}
  	var buf bytes.Buffer
  	if err := errorBodyTemplate.Execute(&buf, errorTemplateInput); err != nil {
  		// Error template MAY NOT fail.
  		panic(errors.Annotate(err, "execution of the error template has failed").Err())
  	}
  	body = buf.String()
  	return
  }
docs: // executeErrorTemplate generates a spartan email that contains information
  // about an error during execution of a user-defined template.