Write Promise::setException in terms of Promise::setTry

Summary: [Folly] Write `Promise::setException` in terms of `Promise::setTry`.

Reviewed By: marshallcline

Differential Revision: D8344928

fbshipit-source-id: 90e21219f054329eca7ae1a6c2ca731a16328b6f

folly/futures/Promise-inl.h

@@ -109,8 +109,7 @@ Promise<T>::setException(E const& e) {
 
 template <class T>
 void Promise<T>::setException(exception_wrapper ew) {
-  throwIfFulfilled();
-  core_->setResult(Try<T>(std::move(ew)));
+  setTry(Try<T>(std::move(ew)));
 }
 
 template <class T>