Summary:
future.then([] -> T)  currently returns Future<T>
future.then([] -> Try<T>)  currently returns Future<Try<T>>

This changes that behavior, such that future.then([] -> Try<T>) will
return Future<T>.

This is a more desirable interface for futures. It also simplifies
implementation of a separate task (propagating exceptions more
efficiently through chained callbacks) because it enables us to internally
substitute lambdas returning T with lambdas returning Try<T>.

Reviewed By: yfeldblum, ryantimwilson

Differential Revision: D14318624

fbshipit-source-id: 51363e82ceb86bfecf87ae661188be9ca3dd8002