xhttp.onreadystatechange = function() {
  if (this.readystate == 4) {
    if (this.status == 200) {
      if (xhttp.responseType == 'blob') {
        // existing goofy file download code here
      } else {
        // report internal error
      }
    } else if (this.status = 404) {
      // report error here
    }
  }
}