Symfony 2 used FormErrorsSerializer.php with jQuery.

form.js

function form_callback(target)
{
  target = $(target);
  $.ajax({
    url: target.attr('action'),
    type: target.attr('method'),
    data: target.serialize(),
    dataType: 'json',
    beforeSend: function() {
      //clear errors
      target.find('.alert-error').remove();
      //disable buttons
      target.find('[type="submit"]').attr('disabled', 'disabled');
    },
    success: function(response) {
      target.find('[type="submit"]').removeAttr('disabled');
      if (response.status == 'error') {
        //process errors
        process_errors(target, response.errors);
      } else {
        if (response.status == 'redirect') {
          window.location.href = response.redirect;
        } else {
          //stay down
          if (response.status == 'data') {
            target.html($(response.data));
          }
        }
      }
    }
  });

  return false;
}

function process_errors(target, errors)
{
  if (target.find('.global-errors-container').length > 0) {
    for (var i in errors.global) {
      target.find('.global-errors-container').append(
        '<div class="alert-error">' + errors.global[i] + '</div>'
      );
    }
  }

  for (var field_id in errors.fields) {
    if ($('#' + field_id).length > 0) {
      $('#' + field_id).before(
        '<div class="alert-error">' + errors.fields[field_id] + '</div>'
      );
    }
  }

}