ignore network errors in linger mode

(fixes bug #10077: After observing game in multiplayer got back to
main screen, not to lobby.)

src/multiplayer_wait.cpp

@@ -403,6 +403,7 @@ void wait::process_network_data(const config& data, const network::connection so
 		return;
 	} else if(data.child("scenario_diff")) {
 		LOG_NW << "received diff for scenario... applying...\n";
+		//! @todo We should catch config::error and then leave the game.
 		level_.apply_diff(*data.child("scenario_diff"));
 		generate_menu();
 	} else if(data.child("side")) {

src/playmp_controller.cpp

@@ -285,6 +285,9 @@ void playmp_controller::linger(upload_log& log, LEVEL_RESULT result)
 		//Catch this error here so mp players quitting unexpectedly are not thrown back
 		//to the title screen
 		result = e.result;
+	// Thrown when receiving [leave_game].
+	} catch(network::error& e) {
+		result = QUIT;
 	}
 
 	// revert the end-turn button text to its normal label