ErrCode::toString is needed by desktop/source/app/ unit tests now
Change-Id: Ie7a9e67aca763c958d08f3826bca5df4e0d828f3
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/175769
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins