Validate the options form.
Overrides FilterPluginBase::value_validate
function value_validate($form, &$form_state) {
$values = drupal_explode_tags($form_state['values']['options']['value']);
$uids = $this
->validate_user_strings($form['value'], $values);
if ($uids) {
$form_state['values']['options']['value'] = $uids;
}
}