rest: use http error code to report mac is in use

report 409 conflict if mac is in use

src/rest.c

@@ -3032,6 +3032,7 @@ static int og_cmd_post_client_add(struct og_rest_ctx *ctx)
 		       computer.ip, computer_ip, computer_name, room_name, hwaddr);
 		dbi_result_free(result);
 		og_dbi_close(dbi);
+		ctx->http_error = OG_HTTP_409_CONFLICT;
 		return -1;
 	}
 	dbi_result_free(result);