{% load firstof from future %}
{% if form.non_field_errors %}
  <div class='row form-group'>
    <div class='{% firstof label_class "col-sm-4" %}'></div>
    <div class='{% firstof field_class "col-sm-8" %} alert alert-warning'>
      {{ form.non_field_errors }}
    </div>
  </div>
{% endif %}