add base template for django-mellon error pages (#25526)

fargo/templates/mellon/base.html

@@ -0,0 +1,12 @@
+{% extends "fargo/base.html" %}
+
+{% block extrascripts %}
+  {{ block.super }}
+  {% block mellon_extra_scripts %}
+  {% endblock %}
+{% endblock %}
+
+{% block content %}
+  {% block mellon_content %}
+  {% endblock %}
+{% endblock %}