Custom Form Error in Symfony 5 (form_errors)

While you can add attributes to form_label and form_widget to customise form_errors you have to use a template.

1. Add a twig template "templates/form/form_errors.html.twig"
2. Paste in the below
   {% block form_errors %}
    {% if errors|length > 0 %}
    <ul class="error_list">
        {% for error in errors %}
            <li>{{ error.message }}</li>
        {% endfor %}
    </ul>
    {% endif %}
{% endblock form_errors %}
3. Edit "config/packages/twig.yaml"
4. Add in the template details eg.
   twig:
    default_path: '%kernel.project_dir%/templates'
    form_themes:
        - 'form/form_errors.html.twig'

Based on Symfony 2 instructions found here