Adapt to changes in couchstore_last_os_error

Change-Id: I0ec035cf698c9312ab99207de35a6223ac6c8441
Reviewed-on: http://review.couchbase.org/30256
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>