MORE than a week after NATO planes began to bomb Yugoslavia, the Western world is being forced, inch by unwilling inch, to acknowledge it has made a terrible mistake. Earlier last week, when the first Kosovar Albanians began to stream across the borders of Macedonia, Jamie Shea, the NATO spokesman in Brussels, denounced it in shrill tones calling it the greatest humanitarian tragedy since World War II. He also brusquely rejected the suggestion that NATO's bombing might have triggered the systematic ethnic cleansing now going on in Kosovo. Serbia was solely responsible, he said, and would have to suffer the consequences.