Handle nicely “POST Content-Length of bytes exceeds the limit of” warning

Good day) I’m building a WordPress plugin LMS allowing user to attach their completed home assignments files via AJAX to their answers. Everything works fine, except one scenario when user’s file exceeds the maximum size allowed. In this case my AJAX call returns HTML with following warning:

<b>Warning</b>:  POST Content-Length of XXXXX bytes exceeds the limit of XXXXX bytes in <b>Unknown</b> on line <b>0</b><br />

As this kind of warning generated on string 0 of my script, it will die and ignore any other instructions, leaving me this ugly string as a result of my call.
You basically can live with it, as the final user will not see it (just the AJAX call will not have desired result), but still I want to know if there is a chance to jump it over and handle it nicely?

Ok, what kind of research I’ve already done:

1. I found this tutorial by Andrew Curioso, and tried to make something like this from it:

   if (isset($_SERVER['CONTENT_LENGTH'])) {
       if ($_SERVER['CONTENT_LENGTH'] > (substr(ini_get('post_max_size'), -1) * 1024 * 1024)) {
           echo 'php errors is so much fun';
       }
   }
   

It doesn’t give the desired effect, as my script still dies with only positive effect of echoing additional string from IF statement (besides, if you try to do something like wp_send_json(errorsArray), it will not be executed).

1. Turning off displaying errors with display_errors, error_reporting. Well, it’s not what I need here, as it still does not allow me to proceed with script and create custom error handler.

2. My WP_DEBUG is set to FALSE

What kind of advice I’m not looking for is manually editing max_upload_size, max_post_size etc. As manually editing server files is out of plugin philosophy. And anyway even if you set your max upload size to 10GB, once you will have final user trying to upload 11GB.

SO, to summarize, as we all know, this mechanism is realized on thousands of sites and apps, and I want to know how to maximize my script quality, handling this issue without overkills and bad UX.

Thank for sharing your thoughts 🙂

UPD1
Here is my AJAX call if it helps:

$('#post-assignment-answer').on('click', function (event) {
    event.preventDefault();

    tinymce.triggerSave();
    var answerContent = tinymce.get('assignment_user_input_textarea').getContent();

    var answerFile = $('#assignment-file-upload')[0].files[0];

    var formData = new FormData();

    formData.append('action', 'post_assignment_answer');
    formData.append('security', lucid_single_assignment_params.post_assignment_answer_nonce);
    formData.append('post_id', lucid_single_assignment_params.post_id);
    formData.append('answer_content', answerContent);
    formData.append('answer_file', answerFile);

    $.ajax({
        url: lucid_single_assignment_params.ajax_url,
        type: 'post',
        data: formData,
        contentType: false,
        processData: false,
        success: function (result) {
            console.log(result);
        }
    });
});

UPD2:
Added AJAX handling php

<?php

/**
 * Post Assignment Answer
 *
 * @version       1.0.0
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit;
} // Exit if accessed directly

// Create AJAX call $result
$result = array(
    'fileTypeError' => false,
    'fileSizeError' => false,
    'uploadError' => false
);

// Retrieving current post ID
$post_id = $_POST['post_id'];

// Retrieving WYSIWYG content
$answer_content = '';

if (! (trim($_POST['answer_content']) == '') ) {

    $answer_content = wp_kses_post($_POST['answer_content']);

}

// Updating WYSIWYG meta field
update_post_meta($post_id, '_answer_content', $answer_content);

// Check if user intends to upload a file
if (!empty($_FILES['answer_file'])) {

    // Adding timestamp to file name
    $_FILES['answer_file']['name'] = round(microtime(true)) . '_' . $_FILES['answer_file']['name'];
    $answer_file = $_FILES['answer_file'];

    // Setting up uploaded file type validation
    $supported_types = array(
        'text/plain' // .txt
    );

    $arr_file_type = wp_check_filetype(basename($answer_file['name']));

    $uploaded_type = $arr_file_type['type'];

    // Setting up uploaded file size validation // TODO: This should be optimized
    $allowed_size = 8388608;
    $uploaded_size = $answer_file['size'];

    // Validating, and in a case of success completing upload
    if (!in_array($uploaded_type, $supported_types)) {

        $result['fileTypeError'] = __('The type of file you've provided is not allowed', 'lucidlms');

    } elseif ($uploaded_size > $allowed_size) {

        $result['fileSizeError'] = __('The size of file you've provided is exceeding the maximum upload size', 'lucidlms');

    } else {

        /**
         * Override the default upload path.
         *
         * @param   array   $dir
         * @return  array
         */
        function lucidlms_assignment_upload_dir( $dir ) {
            global $user_ID;

            return array(
                'path'   => $dir['basedir'] . '/lucidlms/assignment/user' . $user_ID,
                'url'    => $dir['baseurl'] . '/lucidlms/assignment/user' . $user_ID,
                'subdir' => ''
            ) + $dir;
        }

        // Register path override
        add_filter( 'upload_dir', 'lucidlms_assignment_upload_dir' );

        $upload = wp_handle_upload($answer_file, array( 'test_form' => false ));

        // Set everything back to normal
        remove_filter( 'upload_dir', 'lucidlms_user_upload_dir' );

        if (isset($upload['error']) && $upload['error'] != 0) {

            $result['uploadError'] = sprintf(__('There was an error uploading your file. The error is: %s', 'lucidlms'), $upload['error']);

        } else {

            // Check if there is an old version of file on the server and delete it
            $existing_answer_file = get_post_meta($post_id, '_answer_file', true);
            if (! empty($existing_answer_file)) {

                global $user_ID;
                $upload_dir = wp_upload_dir();
                $existing_answer_file_name = pathinfo($existing_answer_file['file'], PATHINFO_BASENAME);
                $unlink_path = $upload_dir['basedir'] . '/lucidlms/assignment/user' . $user_ID . '/' . $existing_answer_file_name;
                unlink($unlink_path);

            }

            // Updating post meta
            update_post_meta($post_id, '_answer_file', $upload);
        }
    }
}

wp_send_json($result);