append_t: Newer server build returns ENOENT rather than NOT_STORED

The latest builds of the server return this error code (which is
actually more correct anyway).

Change-Id: I47390e048857b92113b01bf11dab9909bf31732c
Reviewed-on: http://review.couchbase.org/38222
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>