rgw/rest: RGWRESTConn::forward() prefers to return http errors

rgw/rest: RGWRESTConn::forward() prefers to return http errors

callers need to distinguish between transport errors (a failure to
forward the request) and http errors (successfully forwarded and got a
response). forward() was losing this information by mapping any http
errors to errnos

use tl::expected to differentiate between transport errors and http
errors, with the latter being the successful/expected case

Signed-off-by: Casey Bodley <cbodley@redhat.com>