Open messages in Bootstrap modal
This can be done by twig template and jquery. Add only 'Status messages' block on 'Highlighted' region. and in the twig template add like below: 

{% if page.highlighted %}
          {% block highlighted %}

            &lt;div class="modal fade" id="modal-div"&gt;
              &lt;div class="modal-dialog" role="document"&gt;
                &lt;div class="modal-content"&gt;
                  &lt;div class="modal-header"&gt;
                    &lt;button type="button" class="close" data-dismiss="modal" aria-label="Close"&gt;
                      &lt;span aria-hidden="true"&gt;&amp;times;&lt;/span&gt;
                    &lt;/button&gt;
                    &lt;h4 class="modal-title"&gt;Message&lt;/h4&gt;
                  &lt;/div&gt;
                  &lt;div class="modal-body"&gt;
                    {{ page.highlighted }}
                  &lt;/div&gt;
                  &lt;div class="modal-footer"&gt;
                    &lt;button type="button" class="btn btn-secondary" data-dismiss="modal"&gt;Close&lt;/button&gt;
                  &lt;/div&gt;
                &lt;/div&gt;
              &lt;/div&gt;
            &lt;/div&gt;

          {% endblock %}
        {% endif %}


After that in jquery file :

$(window).load(function(){
    if($.trim($("#modal-div .region-highlighted").text()) != ''){//region will be printed even if the highlighted area is blank
        $('#modal-div').modal('show');
    }
});


This will show your messages in modal window.