Demand weak at 30-year bond auction * Stocks, bonds fall after auction * Rising yields risk higher rates throughout economy (Recasts, adds quotes) By Burton Frierson NEW YORK, May 7 (Reuters) - A U.S. government auction of 30-year bonds met a dismal reception on Thursday, driving down bond and stock prices and raising fears the United States may face difficulty financing spending to stimulate the economy. The $14 billion auction met below-average demand from investors, who forced the government to pay a higher yield. An extended trend of rising yields could force up longer-term interest rates throughout the economy. It...