Form element validation handler for 'Name' field in form_test_validate_required_form().
function form_test_validate_required_form_element_validate($element, &$form_state) {
// Set a custom validation error on the #required element.
if (!empty($element['#required_but_empty']) && isset($element['#form_test_required_error'])) {
form_error($element, $element['#form_test_required_error']);
}
}