Fix /404/ testing page

Author: humitos

readthedocs/core/views/__init__.py

@@ -107,8 +107,14 @@ def server_error_500(request, template_name='500.html'):
     return r
 
 
-def server_error_404(request, exception, template_name='404.html'):  # pylint: disable=unused-argument  # noqa
-    """A simple 404 handler so we get media."""
+def server_error_404(request, exception=None, template_name='404.html'):  # pylint: disable=unused-argument  # noqa
+    """
+    A simple 404 handler so we get media.
+
+    .. note::
+
+        Marking exception as optional to make /404/ testing page to work.
+    """
     response = get_redirect_response(request, path=request.get_full_path())
     if response:
         return response