$this->data['breadcrumbs'][] = array(
'text' => $this->language->get('text_error'),
'href' => $this->url->link('product/product', $url . '&product_id=' . $product_id),
'separator' => $this->language->get('text_separator')
);
<?php foreach ($breadcrumbs as $breadcrumb) { ?>
<?php echo $breadcrumb['separator']; ?><a href="<?php echo $breadcrumb['href']; ?>"><?php echo $breadcrumb['text']; ?></a>
<?php } ?>