How for Plupload to generate and transmit an error on the server side?

Question

JSON

I am writing an engine for uploading files using the Plupload library. I can't figure out how (in what form) to give responses to the client about a file upload error. Errors are generated on the server side if the uploaded file is larger than a certain size or if the file extension is not allowed. I realized that plupload receives the result in the form of a JSON response according to the JSON RPC 2.0 protocol, but I just can’t figure out which particular line it needs to feed in the end. Who thread solved this problem?