Fix unused variable warnings

The forth in my quest to make Godot 3.x compile with -Werror on GCC7

core/io/file_access_network.cpp

@@ -87,6 +87,8 @@ void FileAccessNetworkClient::_thread_func() {
 
 		DEBUG_PRINT("SEM WAIT - " + itos(sem->get()));
 		Error err = sem->wait();
+		if (err != OK)
+			ERR_PRINT("sem->wait() failed");
 		DEBUG_TIME("sem_unlock");
 		//DEBUG_PRINT("semwait returned "+itos(werr));
 		DEBUG_PRINT("MUTEX LOCK " + itos(lockcount));