MB-24489: Error out in case of invalid slice index
MB-24489: Error out in case of invalid slice index

+ Rather than panicing on 'slice bounds out of range'
  while browsing through a non moss directory, error
  out with invalid slice index.

Change-Id: I7dfceef666baabfea2bbbb473e3ff2be84a076dc
Reviewed-on: http://review.couchbase.org/78420
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

couchbaselabs/moss,steveyen/moss,steveyen/moss