MB-10155 Log OS error when CouchKVStore openDB fails

Currently we only log a generic "open failed" error.
Logging OS error gives us more info about the failure.

Change-Id: I5f84ee98dc8717fc7cd955573a53749e7da4ccd8
Reviewed-on: http://review.couchbase.org/36247
Tested-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>