Attempt to fix infinite loading screen if server restarts during MP login

See #2927.

src/game_initialization/multiplayer.cpp

@@ -104,6 +104,10 @@ std::pair<wesnothd_connection_ptr, config> open_connection(std::string host)
 			return std::make_pair(std::move(sock), config());
 		}
 
+		if(!sock->socket_open()) {
+			throw wesnothd_error("The server has shut down or restarted.");
+		}
+
 		data.clear();
 		sock->wait_and_receive_data(data);
 

src/wesnothd_connection.cpp

@@ -186,7 +186,7 @@ void wesnothd_connection::send_data(const configr_of& request)
 void wesnothd_connection::cancel()
 {
 	MPTEST_LOG;
-	if(socket_.is_open()) {
+	if(socket_open()) {
 		boost::system::error_code ec;
 
 #ifdef _MSC_VER

src/wesnothd_connection.hpp

@@ -92,6 +92,11 @@ public:
 	// Destroys this object.
 	void stop();
 
+	bool socket_open() const
+	{
+		return socket_.is_open();
+	}
+
 	/** True if connected and no high-level operation is in progress */
 	bool handshake_finished() const
 	{