core: use postinit status code

frontend/static/js/api/chat/channels.js

@@ -21,8 +21,7 @@ class Channels {
 		});
 
 		let data = await response.json();
-		let exit_code = await response.status;
-		if (exit_code >= 300)
+		if (data.status >= 300)
 			return undefined;
 
 		let messages = undefined;