Processes a form button element.
function form_process_button($element, $form_state) {
// If this is a button intentionally allowing incomplete form submission
// (e.g., a "Previous" or "Add another item" button), then also skip
// client-side validation.
if (isset($element['#limit_validation_errors']) && $element['#limit_validation_errors'] !== FALSE) {
$element['#attributes']['formnovalidate'] = 'formnovalidate';
}
return $element;
}