fix contracts for Future::hasValue() & hasException()
fix contracts for Future::hasValue() & hasException()

Summary:
* Contract for Future::hasValue() had a typo - fixed that.
* Contract for Future::hasException() was inconsistent with hasValue() - made it consistent.

Differential Revision: D9527398

fbshipit-source-id: 93e1ebd768fa38476949818c0902e06a7dab5209

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly