Don't lose parentheses in show SomeAsyncException

I'd also like to change the string-based show instances in this file so that they wrap the strings with spaces in parens when precedence is 11 or higher, but I worry that change might be controversial. But hopefully this simpler change is not controversial, since we just preserves any existing parens in the wrapped exception.

(This PR is copied from GitHub per request of @mpickering: https://github.com/ghc/ghc/pull/253)