Larry Gopnik, the mild-mannered protagonist of Joel and Ethan Coens’ “A Serious Man,” is in a doozy of an existential crisis. It is a Minnesota suburb in 1967, but it’s no Summer of Love for Larry. His wife is leaving him for another man. He is being financially and psychologically squeezed by demanding, self-absorbed children. And just as Larry, who teaches physics at a Midwestern university, is on the cusp of receiving tenure, his colleagues begin getting anonymous letters that accuse him of awful things.