include the request object in build error

so that we will have the URL in sentry

Finds it form the stack frames, as suggested in http://nedbatchelder.com/blog/201008/global_django_requests.html.

Will make errors like http://logs.internal.dimagi.com:9000/commcare-hq-dev-team/hq-production/group/109420/ (requires VPN access + login) more useful