LUN-1869: Since we are managing the state of the current site id, it is ...

...important to sync the CMS_TEMPLATES variables as sooner as possible in the request/response cycle, so that even on early middleware chain breaking (caused by some exception), the error page will be displayed correctly with respect to the current site. This fix also ensures that we are able to show the CMS error page even if there is an early exception raised in the middleware chain
You should add ValueError to the list of exception since cms_admin_site might be '' (empty string).