MB-100: handle ServerUnavailableException in ns_server stats collector

Change-Id: I2444a39880aafae06c7366d6d55dc6a2ddd3db2b
Reviewed-on: http://review.couchbase.org/25468
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>