rgw/multisite: rgw_forward_request_to_master() preserves Error responses

rgw/multisite: rgw_forward_request_to_master() preserves Error responses

when a forwarded request fails on the master zone, the local zone should
return that same error response back to the client. this means
reproducing both the http error and the aws xml <Error> response

rgw_forward_request_to_master() stores these errors in s->err, and
set_req_state_err() now avoids overwriting existing an error

Fixes: https://tracker.ceph.com/issues/71098
Signed-off-by: Casey Bodley <cbodley@redhat.com>