Perhaps the saddest thing about “I Swear,” the moving if also occasionally cloying film about Tourette’s activist John Davidson, is that a movie aimed at forging understanding ended up unwittingly causing division and horror — at a ceremony where it was being celebrated. At the same British Academy Film Awards ceremony in February where Robert Aramayo, the absolutely terrific actor who plays Davidson, was a surprise best actor winner, Davidson’s involuntary racial slur hurled at presenters