Form element validation handler for options element.
function options_field_widget_validate($element, &$form_state) {
if ($element['#required'] && $element['#value'] == '_none') {
form_error($element, t('!name field is required.', array(
'!name' => $element['#title'],
)));
}
// Transpose selections from field => delta to delta => field, turning
// multiple selected options into multiple parent elements.
$items = _options_form_to_storage($element);
form_set_value($element, $items, $form_state);
}