Raise exceptions on HTTP errors.

The undocumented '_http_request' will still take a 'quiet' parameter,
however

Change-Id: Ib68c63583b76e459a496e960e0aad1a54fc92806
Reviewed-on: http://review.couchbase.org/26707
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>