By BART GREENBERG**** Old friends get together for an evening of songs. Two wrote them. Two sing them. Sounds like nothing more than a pleasant evening. But when the two who wrote them are two of the most gifted creators of songs that tell stories and create vivid characters. And the two singers are consummate actors with pleasing voices to tell these tales. There were no mistakes about Beautiful Mistake, the most recent return to Pangea for Austin Pendleton and Barbara Bleier singing the songs of John Bucchino and Amanda McBroom.