eventcally/project/templates/_messages.html

{%- with messages = get_flashed_messages(with_categories=true) -%}
  {% if messages %}
    {% for category, message in messages %}
      <div class="alert alert-{{ category }} alert-dismissible" role="alert">
      <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button>
      {{ message }}
      </div>
    {% endfor %}
  {% endif %}
{%- endwith %}