<?php

/**
 * Implements hook_form_alter().
 */
function modulename_form_alter(&$form, &$form_state, $form_id) {
  // You might want to filter by content type.
  if ($form_id == 'ctype_node_form') {
    // Add a cancel button.
    $form['actions']['cancel'] = array(
      '#type'   => 'submit',
      '#value'  => t('Cancel'),
      '#access' => TRUE,
      '#weight' => 15,
      '#submit' => array('modulename_form_cancel', 'node_form_submit_build_node'),
      '#limit_validation_errors' => array(),
    );
  }
}

/**
 * Custom cancel button callback.
 */
function modulename_form_cancel($form, &$form_state) {
  $url = $_GET['destination'] ? $_GET['destination'] : 'choose/your/path';
  drupal_goto($url);
}