The opening of Neill Blomkamp’s “Chappie” sweeps across multiple stories in multiple places in early stage apocalypse-Johannesburg, South Africa. A helicopter fleet of robocops dives from the sky and blasts through a collection of menacing gangsters. It’s a lovely, frantic movement of action, editing and scoring. And then everything goes wrong after that. It’s common for critics to wish that movie characters would grow brains.