Work-around for move during ponder

Work-around for move during ponder

When engines, in violation of UCI specs, moved during ponder without
waiting for 'stop' or 'ponderhit', UCI2WB would pass their move out
of turn to the GUI, and hang when the opponent would move, waiting
for an engine move that had already come. We now ignore such a
premature ending of the ponder search, like no ponder search was
made at all. And we give an error message in the engine output.