LONDON (Reuters) - Royal Dutch Shell faced a torrent of criticism from analysts on Friday for warning of possible delays to its $25 billion (£19 billion) share buyback programme, with some saying the move had undermined the credibility of the oil giant's management.Shell, the world's second-largest listed oil and gas company, saw its shares close more than 4% lower on Thursday, wiping out $10 billion of its market value. It had earlier reported stronger-than-expected third-quarter profits..