I added this block of code back in from the original Beansteam module in beanstream.php, and all seems to be working again.

Code:
 /**
   * Used to display error message details
   *
   * @return array
   */
  function get_error() {
    $error = array('title' => MODULE_PAYMENT_BEANSTREAM_TEXT_ERROR,
                   'error' => stripslashes(urldecode($_GET['error'])));
    return $error;
  }