So next time I write <data type="ST_Foo"> instead of <ref
name="ST_FOO">, I get an error quickly.

Change-Id: Ida91b8ebe68915e22c62addffafe418ab29b1158
Reviewed-on: https://gerrit.libreoffice.org/59661
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>