Improving error handling for Browser

[swistak35]

Currently, if for some reason fetching of the event from RES Browser API fails, Browser frontend assumes that the event is "not found". We should differentiate between cases when the event does not exist and when there was problem with connection / API server.

[swistak35]

Also, there's no distinction between "event not loaded yet" and "event not found", so sporadically, user may see "Event not found" information for a fraction of second when the event loads.