AVIGNON, France, July 21 (AFP) - This year's edition of one of Europe's top summer arts events was described as a pretentious catastrophe Thursday after angry audiences booed or walked out of a series of performances. Critics attending the three-week Avignon theatre festival in southern France said it had plumbed new depths of intellectual obscurity and warned that a contempt for the mainstream public was placing the future of a prestigious national institution in jeopardy. "What purgatory!" headlined the news magazine Le Point on its culture pages, "Loyal spectators are sad, disorientated and haggard," while a commentator for the Communist...