[release-branch.go1.3] runtime: ignore exceptions from foreign threads.

««« CL 104200046 / 14683b1cf2cc
runtime: ignore exceptions from foreign threads.
Fixes issue 8224.

LGTM=alex.brainman, rsc
R=alex.brainman, rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/104200046
»»»

LGTM=alex.brainman, minux
R=rsc, alex.brainman, minux
CC=golang-codereviews
https://codereview.appspot.com/126010043