send_email: Increase loglevel of stuck ScheduledEmail messages to error.

Since these stay in the queue indefinitely, they deserve to be raised
at a higher error than just warning.

zerver/lib/send_email.py

@@ -434,7 +434,7 @@ def deliver_scheduled_emails(email: ScheduledEmail) -> None:
         # This state doesn't make sense, so something must be mutating,
         # or in the process of deleting, the object. We assume it will bring
         # things to a correct state, and we just do nothing except logging this event.
-        logger.warning("ScheduledEmail id %s has empty users and address attributes.", email.id)
+        logger.error("ScheduledEmail id %s has empty users and address attributes.", email.id)
         return
 
     if user_ids:

zerver/tests/test_users.py

@@ -1468,7 +1468,7 @@ class ActivateTest(ZulipTestCase):
         self.assertEqual(
             info_log.output,
             [
-                f"WARNING:zulip.send_email:ScheduledEmail id {email.id} has empty users and address attributes."
+                f"ERROR:zulip.send_email:ScheduledEmail id {email.id} has empty users and address attributes."
             ],
         )