MB-16181: readCollectionsManifest shouldn't always log a warning
MB-16181: readCollectionsManifest shouldn't always log a warning

It's ok if the manifest document is not present, so don't warn
for not_found error.

Change-Id: I428c2288a58460fffc9c2426fb232721ee559f58
Reviewed-on: http://review.couchbase.org/75273
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,couchbase/ep-engine