How to use Drupal messages within an ajax call.

gistfile1.php

<?php

/**
 * Implements hook_menu().
 */
function MYMODULE_menu() {
  $items = array();

  $items['login'] = array(
    'title' => t('Log in'),
    'page callback' => 'drupal_get_form',
    'page arguments' => array('MYMODULE_login_form'),
    'access callback' => TRUE,
  );

  return $items;
}

//------------------------------------------------------------------------------
// LOG IN FORM
//------------------------------------------------------------------------------

/**
 * Form MYMODULE_login_form callback.
 */
function MYMODULE_login_form() {
  $form = array();

  $form['messages'] = array(
    '#markup' => '<div id="MYMODULE-login-form-messages"></div>',
    '#type' => 'markup',
  );

  // Add other fields here

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Log in'),
    '#ajax' => array(
      'callback' => 'MYMODULE_login_callback',
      'progress' => array(
        'type' => 'throbber',
        'message' => t('Please wait...'),
      ),
      'effect' => 'fade',
    ),
  );

  return $form;
}

/**
 * AjaX callback for the visustock_login_form submission.
 */
function MYMODULE_login_callback($form, $form_state) {
  $commands = array();

  $errors = form_get_errors();
  if (empty($errors)) {

    // Do something here

  }

  return _MYMODULE_build_callback($form, $form_state, $commands);
}

//------------------------------------------------------------------------------
// HELPERS
//------------------------------------------------------------------------------

/**
 * Generic callback
 */
function _MYMODULE_build_callback($form, $form_state, array $commands) {
  $target_id = _MYMODULE_get_target_id($form, $form_state);

  $commands[] = ajax_command_html('#' . $target_id . '-messages', theme('status_messages'), array('effect' => 'fade'));
  $commands[] = ajax_command_changed('#' . $target_id . '-messages');

  return array(
    '#type' => 'ajax',
    '#commands' => $commands,
  );
}

/**
 * Get target id from 
 */
function _MYMODULE_get_target_id($form, $form_state) {
  $target_id = $form['#form_id'];
  if (!empty($form_state['clicked_button']['#ajax']['wrapper'])) {
    $target_id = $form_state['clicked_button']['#ajax']['wrapper'];
  }

  $target_id = strtr(drupal_strtolower($target_id), array(' ' => '-', '_' => '-', '[' => '-', ']' => ''));

  return $target_id;
}