Fix various xhr auto-test failures

In the v8 compatibility layer accessor wrappers, forward v4 exceptions
instead of silently eating them. We don't use the v8 TryCatch layer anymore.

Change-Id: I43596c58b6f51a481a11e33a87d5294f1336471b
Reviewed-by: Lars Knoll <lars.knoll@digia.com>