Fail suspected API requests with 404, not 503

Non-running user servers making requests is a fairly
common occurance - user servers get culled while their
browser tabs are left open. So we now have a background level
of 503s responses on the hub *all* the time, making it
very difficult to detect *real* 503s, which should ideally
be closely monitored and alerted on.

I *think* 404 is a more appropriate response, as the resource
(API) being requested is no longer present.

jupyterhub/handlers/base.py

@@ -1349,7 +1349,7 @@ class UserUrlHandler(BaseHandler):
         self.log.warning(
             "Failing suspected API request to not-running server: %s", self.request.path
         )
-        self.set_status(503)
+        self.set_status(404)
         self.set_header("Content-Type", "application/json")
 
         spawn_url = urlparse(self.request.full_url())._replace(query="")