Form validation handler for node_assign_owner_action_form().
node_assign_owner_action_submit()
function node_assign_owner_action_validate($form, $form_state) {
$exists = (bool) db_query_range('SELECT 1 FROM {users} WHERE name = :name', 0, 1, array(
':name' => $form_state['values']['owner_name'],
))
->fetchField();
if (!$exists) {
form_set_error('owner_name', t('Enter a valid username.'));
}
}