JCBC-266: Harden HTTP Provider on empty pool buckets.

This changeset makes sure a proper exception is thrown when
no valid bucket configuration could be found. The resubscriber
picks it up and retries again.

Change-Id: I254c5498a149fc2bab2f5459b10c0654354d23fe
Reviewed-on: http://review.couchbase.org/26862
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>