missed-message: Add a try-catch to prevent killing background thread.

An exception which escapes from this loop can kill the background
worker thread; this results in consuming the queue (leading to the
illusion of progress) but more and more rows silently piling up in the
ScheduledMessageNotificationEmail table.

Wrap the inside of the `while True` loop in a try/catch to make sure
that no exceptions escape and kill the background thread.  To prevent
even more indentation, the inner loop is extracted into its own
function.  It returns true/false to signal if the `self.stopping` was
set to tell the loop to stop; we cannot check it ourselves in the
outer loop because it needs to hold the lock to be examined.

zerver/worker/queue_processors.py

@@ -657,9 +657,20 @@ class MissedMessageWorker(QueueProcessingWorker):
 
     def work(self) -> None:
         while True:
+            try:
+                finished = self.background_loop()
+                if finished:
+                    break
+            except Exception:
+                logging.exception(
+                    "Exception in MissedMessage background worker; restarting the loop",
+                    stack_info=True,
+                )
+
+    def background_loop(self) -> bool:
         with self.cv:
             if self.stopping:
-                    return
+                return True
             # There are three conditions which we wait for:
             #
             #  1. We are being explicitly asked to stop; see the
@@ -708,6 +719,8 @@ class MissedMessageWorker(QueueProcessingWorker):
         if not was_notified:
             self.maybe_send_batched_emails()
 
+        return False
+
     def maybe_send_batched_emails(self) -> None:
         current_time = timezone_now()