Add error text as body

When Memcached responds with an error, it contains the error as string
in the body of the response. Make the fake UPR server do the same.

Change-Id: I9b2ff8780662c5c83bc4abbbe1ac8f225dbedbb9
Reviewed-on: http://review.couchbase.org/33743
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>