fix ExceptionWrapper comment

fix ExceptionWrapper comment

Summary:
Fix comment for exception_wrapper's `operator bool()`.
It reads "return `true` if `*this` is not holding an exception"
but the actual behavior is to return `true` if `*this` *is* holding
an exception.

Reviewed By: yfeldblum

Differential Revision: D5456386

fbshipit-source-id: ec3a472ea441476d835881e94036a3957bb2ab32