Support stat errors without body

Currently stat errors contain a body, but they don't necessarily
need to. Hence support errors without a body.

Change-Id: I38fc1f516749053feb973053df68d8c86ce04a13
Reviewed-on: http://review.couchbase.org/34432
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>