cache: Decline to store querysets, with an error.

As we have seen no further cases of this in production since #23215,
increase the severity to an error, and switch from returning a
list (which is not type-safe if the function declares a QuerySet
return) to returning the QuerySet without caching.

Failing to store the result in the cache, with an error, seems
superior to raising an exception; in both cases the next request will
redo the work, but we are guaranteed a worse user experience if we 500
the request.

Ref https://github.com/zulip/zulip/pull/23215#discussion_r994186493

zerver/lib/cache.py

@@ -170,12 +170,11 @@ def cache_with_key(
 
             val = func(*args, **kwargs)
             if isinstance(val, QuerySet):  # type: ignore[misc] # https://github.com/typeddjango/django-stubs/issues/704
-                logging.warning(
-                    "cache_with_key attempted to store a full QuerySet object -- flattening using list()",
+                logging.error(
+                    "cache_with_key attempted to store a full QuerySet object -- declining to cache",
                     stack_info=True,
                 )
-                val = list(val)
-
+            else:
                 cache_set(key, val, cache_name=cache_name, timeout=timeout)
 
             return val