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="")

jupyterhub/tests/test_pages.py

@@ -1001,7 +1001,7 @@ async def test_token_page(app):
 async def test_server_not_running_api_request(app):
     cookies = await app.login_user("bees")
     r = await get_page("user/bees/api/status", app, hub=False, cookies=cookies)
-    assert r.status_code == 503
+    assert r.status_code == 404
     assert r.headers["content-type"] == "application/json"
     message = r.json()['message']
     assert ujoin(app.base_url, "hub/spawn/bees") in message