bc_http: invoke provider_failed() on I/O error

Change-Id: I7872d544c08632c0d2d04e45e32cd063b57c31d4
Reviewed-on: http://review.couchbase.org/36548
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>