In grade/import/xml/import.php

$error = '';

$importcode = import_xml_grades($text, $course, $error);

if ($importcode !== false) {

    // ...

} else {

    print_error('error', 'gradeimport_xml');

}

Well, you get some marks for effort. The $error variable is initialised, and import_xml_grades even returns useful information using that argument (passed by reference).

However, that useful information is not displayed!