Handle the 404 encountered when FILTER_VHOST is enabled but the vhost's domain does not exist yet
The response to Mailman's /domains/<mail_host>/lists
if the list domain in mail_host
does not exist is 404. With FILTER_VHOST
enabled, this would cause the Postorius root page for unauthenticated clients (or authenticated clients with no roles) to raise an exception if the vhost's domain did not exist yet.