Don't just tell the problem but hint how to fix it.

Change-Id: I9d079ee7d4ed61266e22a3fa21efe10366724645
Reviewed-on: https://gerrit.libreoffice.org/49471Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>