Rename disconnected event to network-error event for clarity

Did this as part of an ultimately unnecessary approach to <https://trello.com/c/ZdhUntrs/319-when-i-signout-i-should-not-see-a-red-bar-stating-you-need-to-sign-in-so-talkilla-doesn-t-feel-overbearing>.  Offering it as a way to make the current code a bit clearer until we overhaul the signin / signout architecture. If it's not worth taking, I'm ok with that too.