Wrap most of fetch_events in a try/except JsonableError.

This is a no-op right now, but we'll want the new structure for the
next commit, and splitting this out makes it a lot easier to read what
is actually changed in the next commit.

zerver/lib/event_queue.py

@@ -447,6 +447,7 @@ def fetch_events(query):
     client_type_name = query["client_type_name"]
     handler_id = query["handler_id"]
 
+    try:
         was_connected = False
         orig_queue_id = queue_id
         extra_log_data = ""
@@ -481,6 +482,9 @@ def fetch_events(query):
         if was_connected:
             logging.info("Disconnected handler for queue %s (%s/%s)" % (queue_id, user_profile_email,
                                                                         client_type_name))
+    except JsonableError as e:
+        raise e
+
     client.connect_handler(handler_id, client_type_name)
     return (RespondAsynchronously, None)