How do you send a message to the client when PHP form submission download fails

My app has the ability to request and download a multitude of different files and types from the database.

The way I'm currently doing this is with a hidden form input that will submit the request to server which then does the processing and downloads the file. The code for that looks like this:

       var action = '/download/downloader.php?actionname&filename=' + downloadfilename;
        
        var A = document.createElement("div");
        A.style.display = "none";
        document.body.appendChild(A);
        var formKey = 'someid';
        A.innerHTML = '<form id="' + formKey + '" method="post" action="' + action + '" accept-charset="utf-8"  enctype="application/x-www-form-urlencoded"' + target + '><input type="hidden" name="segmentno" id="segmentno"/> </form>';
        document.getElementById(formKey).submit();
        A.parentNode.removeChild(A);

The PHP side for downloading an XML file will look something like this (all the cases set the output data and headers accordingly)

// $a is the data from the database
        $xmlEncode = new XMLEncode();
        $xmlstring =  $xmlEncode->XML_SerializeData($a);
        $xmlstring = str_replace(''', '''', $xmlstring);
    
        header("Content-type: application/xml; charset=UTF-8");
        header("Content-Disposition: attachment;filename=". $this->sFileName .".xml");
        header("Cache-Control: max-age=0");

        exit($xmlstring);
    

This works great when there is no pre-processing required (permission checks, valid key checks, etc...)

However, if I have some server-side pre-processing to do prior to this portion, and it fails, I need a way to send a message back to the front end.

I know with what I have, it will not work. My only thought is that I will need to do the pre-processing in a separate AJAX call and then if it's successful, run the actual download but that isn't ideal. Some other solutions I've looked into include using fetch to get the data back and then potentially using a library to download the blob data but again, I'm not sure if I can tell whether or not it was actually a successful server request or not.

Thanks in advance.