service/app/templates/flash_messages.html

{% with messages = get_flashed_messages(with_categories=true) %}
{% if messages %}
{% for category, message in messages %}
<div class="alert alert-{{ category }}" role="alert">
    {% if category == 'danger' %}
    <span class="glyphicon glyphicon-exclamation-sign" aria-hidden="true"></span>
    {% endif %}
    <span class="sr-only">{{ category }}:</span>
    {{ message }}
</div>
{% endfor %}
{% endif %}
{% endwith %}