detect_404_before_error

// https://www.finleyghosh.com/blog/check-for-a-page-404-with-only-javascript

function UrlExists(url) {
    var http = new XMLHttpRequest();
    http.open('HEAD', url, false);
    http.send();
    if (http.status != 404)
        return true;
    else
        return false;
}

if ( UrlExists(selectedTextPath) ) {
  // load external file.

} else {
  // don't load.
}