The challenge for Berlin detective Bernie Gunther couldn’t be more extreme: Nazi Germany, where he tries to dispense a rough justice amid so much officially sanctioned crime. In “Prague Fatale,’’ a Nazi officer suspects an assassin is plotting an attack at the country estate he has commandeered. A murder does take place ­there — not of the officer but an addled young adjunct who has confided to Gunther of his torment over executing Jews and other innocent civilians.