CBQE-1696: RestConnection.__init__ should raise ServerUnavailableExceptioin

Change-Id: If0717d197fdb1c459a761c8958f814b04c6a6c90
Reviewed-on: http://review.couchbase.org/30283
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>