MB-11019: notify users during auth that server is down

Change-Id: I2954a6dcb0f773983d7f32590ff1fe0bc1480921
Reviewed-on: http://review.couchbase.org/37441
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>