MB-10939 start_cluster doesn't return pid anymore.

So adapted its callers to not expect it. Also improved error handling
in start_cluster itself. Because in some places we assume that if
anything bad happens it just returns an error, but doesn't crash.

Change-Id: Ic36350bfa0e0fbe16f2e1bec376974a58daacd0f
Reviewed-on: http://review.couchbase.org/36237
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>