Welcome to the opening night of “The Murder at Haversham Manor,” where things go from not very good to an utter disaster. “The Play That Goes Wrong” is an ill-fated play-within-a-play, set on the opening night of the Cornley Drama Society’s newest production. The troupe’s ambitious whodunit has everything you never want in a show: falling props, collapsing scenery, an unconscious leading lady, a corpse that can’t play dead and actors who trip over everything … including their lines.