Also add error message to make this more clear in the future.

Change-Id: Ic9e3a2c89119ef1ec3e6cc1074b7419f7ee268b7
Reviewed-on: https://gerrit.libreoffice.org/29375Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>