Walking around D.C. you sometimes catch posters for shows at the Black Cat or 9:30 Club hung up in random places. They always seem a bit ragged and most often ripped up from the elements or from bored hands. Unfortunately, the street isn’t the most forgiving gallery, but silk-screened gig posters do bring art to the masses. Sweet: The Graphic Beauty of the Contemporary Rock Poster brings this medium in from the cold and displays…