Why are art films becoming horror films? Perhaps art film directors are finding that the most effective way to relate to our frazzled age is to mask it in the aesthetics of terror. Last year’s apocalyptic ballet movie, Darren Aronofsky’s Black Swan, heralded this new trend--that movie might as well have had zombie dancers. This year it’s Take Shelter, Jeff Nichols' story of mental illness, marriage, and prophecies of doom. Shelter stars