write-behind: preserve error returned as-is

Change-Id: Ib766403774c1323e0bbddafedeaa47e7fa3a59fa
Signed-off-by: Amar Tumballi <amarts@redhat.com>
BUG: 987415
Reviewed-on: http://review.gluster.org/5296
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>