Only send 500 responses on exceptions when safe

In particular:

If the user has already sent (at least) the beginning of a request, and
the response has not yet started.

Conflicts:

	warp/Network/Wai/Handler/Warp/Run.hs
Disable some exception tests

Conflicts:

	warp/Network/Wai/Handler/Warp/Run.hs
Fix code
Version bump