wagtail_bakerydemo/bakerydemo/jinja2/includes/messages.html

{% if messages %}
    <div class="alert">
        <p class="alert__title">Error</p>
        <ul class="alert__messages">
            {% for message in messages %}
                <li{% if message.tags %} class="{{ message.tags }}"{% endif %}>{{ message }}</li>
            {% endfor %}
        </ul>
    </div>
{% endif %}