More obvious failure if SITE_ID is non-int

* get_cms_settings now raises an ImproperlyConfigured exception if
  SITE_ID is not an integer (when 'LANGUAGES' is accessed).
* `cms check` now shows an error if SITE_ID is not an integer
Merge pull request #2584 from ojii/handle-non-int-site-ids-better

More obvious failure if SITE_ID is non-int