Bootstrap 3 form_errors block

bootstrap_3_layout.html.twig

{% block form_errors -%}
    {% if errors|length > 0 -%}
    {% if form is not rootform %}<span class="help-block">{% else %}<div class="alert alert-danger">{% endif %}
    <ul class="list-unstyled">
        {%- for error in errors -%}
            <li><span class="glyphicon glyphicon-exclamation-sign"></span> {{ error.message }}</li>
        {%- endfor -%}
    </ul>
    {% if form is not rootform %}</span>{% else %}</div>{% endif %}
    {%- endif %}
{%- endblock form_errors %}