NCBC-606: Catch and log ServerUnavailableException during failover

Change-Id: Ie0f74d2bf69cd43ea399762fc39e7ca7f305c11c
Reviewed-on: http://review.couchbase.org/41384
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>