add file as initializer

field_with_error.rb

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  html = %(<span class="has-error">#{html_tag}</span>).html_safe
  form_fields = %w(textarea input select)
  element = Nokogiri::HTML::DocumentFragment.parse(html_tag).css(['label', form_fields].join(', ')).first

  if element.node_name.eql?('label')
    html = %(<span class='has-error'>#{element}</span>).html_safe
  elsif form_fields.include?(element.node_name)
    html = %(<span class='has-error'>#{element}<span class='help-block'>#{[instance.error_message].join(', ')}</span></span>).html_safe
  end

  html
end