We mark DBG_PRINTEXCEPTION_C messages in VEH handler
as handled, thus preventing debugger from seeing them.
I don't see reason for doing that. The comment warns
of crashes, but I added test and don't see any crashes.

This is also simplify VEH handler before making
changes to fix issue 8006.

Update #8006

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/146800043