Improve error messages when qhp files are messed up

Hopefully this saves me and everyone time when mixing up project names
resulting in empty namespaces in .qhp projects.

Change-Id: Ib4151225011c866219504c1fdbaadd17d387500a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>