LUN-1544: If the current site (active site of a certain session) is deleted...

... by another user, the user needs to be notified in a nice way (HTTP 404) that the site is not there anymore
I doubt it's better to do like that. Imagine a similar situation:

def open(file_path):
     if open() syscall returns -1 and errno == ENOENT
         raise CannotDoThing  # wouldn't it be better to raise FileNotFound?
    #... return fd, etc


def something():
    try:
         f = open(file_path)
   except CannotDoSomething:
         dont_do_something()
   do_something()

I consider it's better that the read function returns FileNotFound.
Here, likewise, I think a function which needs to return a list of templates for a given site_id should raise a SiteNotFound. The 404 thing is the responsibility of the caller. And in this case the caller deals with request / responses, so it's its duty to break the response cycle and raise a 404.

Btw, I changed the name of the get_restricted_instances(), since it says nothing about what it does.