Proxy - saveAll method

https://snippet.webix.com/fypwd2g5
Hello! There is a problem when I use a proxy. If I implement the saveAll method, then I get an error when the response status is> = 400.

I think this is due to the next line:
url.saveAll(this.config.master, to_send, this, this._send_callback({}));
(webix UI v.5.4.0 GPL > line 34970)

By default status code >= 400 is processed as an error.

Is there some error codes in that range, those are not errors?

@maksim
Excuse me. I probably did not exactly explain the problem.
When the response status is> = 400, the application crashes.