Exception handling for failing exit node selection

In context of tpo/applications/vpn#38 (closed), the client/tor-vpn should be informed if a selected exit node country doesn't provide any internet connectivty, e.g. since there's no exit node available in the respective country. This could be implemented as a specific exception that gets thrown in this case. These exceptions could be thrown in refreshCircuits(), refreshCircuitsForApp() and startProxy().