Remove Promise::getFuture from Future-inl.h 3/4 - onError
Remove Promise::getFuture from Future-inl.h 3/4 - onError

Summary: Replaces calls to Promise::getFuture in onError calls. Modifies the returned future to complete on the same executor as the caller. No assumption that the future already has an executor (though it should in principle, the codebase is not ready for that yet).

Reviewed By: yfeldblum

Differential Revision: D7104850

fbshipit-source-id: ea152102f8ecd612e322b98a54d9440495b14bf8

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