Git commits in Moodle 3.9.0 by Zig Tan

commit 13954424748a4e7c8f3240f2da6e74c531c10eae
Author: Zig Tan <zig@moodle.com>
Date:   Thu, 11 Jan 2018 13:44:42 +0800
Tag:    v3.9.0-beta

MDL-54554 files: Improve unoconv error logging & sanity checks