Test the class which represents exception_t against true before running unhandled exception handlers. * This allows cheap cancellation of the control flow by passing   a default constructed `exception_t` object to `set_exception`. * Since possible representatives like     - `std::exception_ptr`     - `std::error_code`     - `std::error_condition`   can be default constructed we have to test them anyway before possibly   rethrowing them.
Test the class which represents exception_t against true before running unhandled exception handlers.
* This allows cheap cancellation of the control flow by passing
  a default constructed `exception_t` object to `set_exception`.
* Since possible representatives like
    - `std::exception_ptr`
    - `std::error_code`
    - `std::error_condition`
  can be default constructed we have to test them anyway before possibly
  rethrowing them.

Naios/Continuable