"The Courier," a Cold War spy thriller starring Benedict Cumberbatch, is earnest to a fault. Meaning it's unaccountably dull, even though Cumberbatch gives another of his strong performances, this time in the role of Greville Wynne, a British civilian recruited by MI6 and the CIA to travel to the Soviet Union and make covert contact with a Soviet intelligence officer who wants to help the West avoid a nuclear war.