rgw: when failing read from client, return correct error

Fixes: #6214
When getting a failed read from client when putting an object
we returned the wrong value (always 0), which in the chunked-
upload case ended up in assuming that the write was done
successfully.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>