Fix exception logging in sendalerts

The on_notify_done callback was accessing exception data incorrectly.
If there's been an exception in the thread, it will be re-thrown while
calling future.result(), and we must catch it with "try ... except"
instead of calling future.exception().

hc/api/management/commands/sendalerts.py

@@ -79,10 +79,11 @@ class Command(BaseCommand):
     def on_notify_done(self, future: Future[str | None]) -> None:
         close_old_connections()
         self.seats.release()
-        if logs := future.result():
-            self.stdout.write(logs)
 
-        if exc := future.exception():
+        try:
+            if logs := future.result():
+                self.stdout.write(logs)
+        except Exception as exc:
             logger.error("Exception in notify", exc_info=exc)
             raise exc