Replace Future::onError with Future::thenError
Replace Future::onError with Future::thenError

Summary:
Replace Future::onError with Future::thenError:
 * to remove ambiguous typing
 * to ensure that the executor is not lost and the returned Future is still bound to an executor

See:
https://fb.workplace.com/groups/fbcode/permalink/2002251863144976/
for details.

Reviewed By: yfeldblum

Differential Revision: D14962049

fbshipit-source-id: f46032b9e5f6b740e69a1828e31c9de7b9235b1e

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro