field_error_proc override to add the "has-error" class

environment.rb

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  if html_tag.match(/class=(\'|\")([\w\s\d]*?)\1/i)
    html_tag.gsub(/class=(\'|\")([\w\s\d]*?)\1/i, 'class="\2 has-error"')
  else
    html_tag.gsub(/\<[\w]+/i, '\0 class="has-error"')
  end
end