Form validation handler for contact_site_form().
function contact_site_form_validate($form, &$form_state) {
if (!$form_state['values']['cid']) {
form_set_error('cid', t('You must select a valid category.'));
}
if (!valid_email_address($form_state['values']['mail'])) {
form_set_error('mail', t('You must enter a valid e-mail address.'));
}
}