readwrite-easy: Don't invoke error callback if bytes were read

This allows us to swallow the entire read buffer contents and only
complain if we've had a close WITHOUT data.

Change-Id: Ib7601d696e826af13ce35cdec5d9abf450a25303
TODO: Maybe place this inside sockrw_v0 read itself?
Reviewed-on: http://review.couchbase.org/32914
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>