client: output response on sharing error, for easier debugging

Context: http://camlistore.org/issue/462

Change-Id: Iea68138fb2b5cfeb66773a83dc84ebbcd6442f44
Merge "client: output response on sharing error, for easier debugging"
schema: use existing constant instead of literal

Change-Id: Ia988f4c897788be696326de9a75973161ae3d550