Symfony - Setting Flash and Checking in TWIG

In Controller

$this->get('session')->getFlashBag()->set('error', 'Does Not Exist');

or 

$this->get('session')->getFlashBag()->add('error', 'Does Not Exist');

In Twig

{% for flashMessage in app.session.flashbag.get('error') %}

    {{ flashMessage }}

{% endfor %}

FYI: Doc

In Controller :

$this->get('session')->getFlashBag()->add('error', "User does not exists.");

In View :

{% for type, messages in app.session.flashbag.all() %}
    {% for message in messages %}
        {% if type == 'error' %}
            {{ message }}
        {% endif %}
        {# Or even with css class rendering:
            <div class="flash-{{type}}">{{message}}</div>
        #}
    {% endfor %}
{% endfor %}