Service Discovery sends browser error on chatrooms to GC tabs of rooms you are in
Error from browsing is dispatched as any other error it passes to handle_event_error_answer() where is gajim checking if sender is in self.windows[account]['gc'] [and it is, because error is send by room which doesn't support jabber:iq:browse ]
I think that it shouldn't be displayed here. It confused me when I've looked there 10 minutes later, not knowing anything about service discovery ;)