templates: add base mellon template for error pages (#40987)

templates/mellon/base.html

@@ -0,0 +1,11 @@
+{% extends theme_base %}
+
+{% block extra_scripts %}
+   {% block mellon_extra_scripts %}
+   {% endblock %}
+{% endblock %}
+
+{% block content %}
+   {% block mellon_content %}
+   {% endblock %}
+{% endblock %}