rgw: rework error handling a bit

rgw: rework error handling a bit

Rados Gateway: get rid of RGWOp::err. We already have req_state::err and
that represents the same thing.

Standardize nomenclature for errors. 'errno' is our internal
representation of the error. 'code' is what is returned by S3.
'message' is the message at the end. Improve rgw_err.

dump_errno shouldn't modify req_state, but just dump the error.
A new function set_req_state_err sets the error based on an 'errno'.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>