def errors_for(field_name)
      error_messages = errors[field_name]
      if error_messages.present?
        @template.content_tag :span, class: :error do
          error_messages.join(@template.tag :br).html_safe
        end
      else
        nil
      end
    end
def errors_for(field_name)
      error_messages = errors[field_name]
      if error_messages.present?
        @template.content_tag :span, class: :error do
          error_messages.join(@template.tag :br).html_safe
        end
      else
        nil
      end
    end
Renders a +<span>+ with errors if there are any for the specified field, or returns +nil+ if not.

 @param field_name [String, Symbol] Name of the field to check for errors
 @return [SafeBuffer, nil] The rendered HTML or nil