Committee List button inoperative if sever off during startup

[vteague]

Current behaviour: If the server is off or unreachable during startup, the app can't find the committees. Then, when the user clicks the 'committee' button in the raising screen, it just hangs.

Expected behaviour: Fail elegantly like we do for MPs. Consider whether it's better to ship with a pre-loaded list of committees, which may rapidly go out of date, or simply disable the button if we don't have them, or if we should try again to reach the server if it was initially offline.