Only add a failover log if the stream was succesfully created

Calling the failover callback can actually cause two responses to be
sent in the case of stream request failure. This can cause clients
to become confused and drop connections.

Change-Id: Ibe1b7116b0424c048144682ebe43a3f204e6e4be
Reviewed-on: http://review.couchbase.org/31584
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>