tornado: Use a machine-readable error code when an event queue is gone.

This fixes the original issue that #5598 was the root cause of; when
the user returns to a Zulip browser tab after they've been idle past
the timeout (10 min, per IDLE_EVENT_QUEUE_TIMEOUT_SECS), we now
correctly reload the page even if they're using Zulip in German or
another non-English language where we have a translation for the
relevant error message.

zerver/lib/exceptions.py

@@ -35,6 +35,7 @@ class ErrorCode(AbstractEnum):
     QUOTA_EXCEEDED = ()
     BAD_NARROW = ()
     UNAUTHORIZED_PRINCIPAL = ()
+    BAD_EVENT_QUEUE_ID = ()
 
 class JsonableError(Exception):
     '''A standardized error format we can turn into a nice JSON HTTP response.