Shrink the implementations of then and onError
Shrink the implementations of then and onError

Summary:
[Folly] Shrink the implementations of `then` and `onError`.

Just a small refactor.

Reviewed By: spacedentist

Differential Revision: D5180784

fbshipit-source-id: a399d18500a2b4c5d8f24dee54891cca802b4461

Orvid/folly,facebook/folly,facebook/folly,Orvid/folly,Orvid/folly,facebook/folly,rklabs/folly,facebook/folly,Orvid/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,rklabs/folly