luci-app-sqm: fix JS error if service is disabled

* fix a 'resource not found error' if the sqm service has been disabled
  and the dir '/var/run/sqm/available_qdiscs' not exist
* inform the user about this issue and provide a button
  to enable & start the service

Signed-off-by: Dirk Brenken <dev@brenken.org>