app/templates/formerrors.html

{% if form.errors %}
<div class="alert alert-danger" role="alert">
    {% for field in form %}
    {% for message in field.errors %}
    <span class="glyphicon glyphicon-exclamation-sign" aria-hidden="true"></span>
    <span class="sr-only">Error:</span>
    {{ message }} <br>
    {% endfor %}
    {% endfor %}
</div>
{% endif %}