add a crash handler at initialization

So then I think my original comments apply:

* crash is a bad term for an individual child thread dying.
* For initialization errors, it would be better to use `catch` explicitly.

That said, I'm not convinced that this changed default is a good thing. Any threads forked by Warp should never die with exceptions (since Warp should catch those exceptions itself and use its own exception handler), and for threads forked in other ways: do we really know better than GHC what the default should be?