Summary:
* `BadExpectedAccess<void>` inherits `std::exception` and `BadExpectedAccess<E>` inherits `BadExpectedAccess<void>`.
* Copy-constructor, copy-assignment-operator.
* Member `error` returns a reference.

Paper: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0323r11.html.

Reviewed By: Gownta

Differential Revision: D33907293

fbshipit-source-id: 5002dfa548840431f957540bec23d95f34c9a68d