Summary:
Given an exception_wrapper `ew` holding a `std::runtime_error`, the following assertion fails:

```
try {
  ew.handle();
} catch( std::runtime_error const & ) {
} catch( std::exception const & ) {
  assert(false);
}
```

Reviewed By: simpkins

Differential Revision: D9199924

fbshipit-source-id: b6f90ccf269a1b58f0dd07723451ff34cde1d529