Errol Morris’s conversation with the late David Cornwell in “ The Pigeon Tunnel ” is fascinating even without the filmmaking flourishes. Cornwell, better known by his pen name John le Carré, was the spy-turned-novelist whose tales of espionage and betrayal defined an era, gave literary heft to a genre and inspired numerous adaptations. And here, Morris challenges him to reflect on his unconventional childhood — his con-man father, his mother who left him at 5 never to return — on through his time in the secret services and beyond.