improve gbuild-to-ide error message

Change-Id: Ia8986764e48f1dbe39374d511a188bd2b5f49e13
Reviewed-on: https://gerrit.libreoffice.org/11654
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>