sync experimental with 4999

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@5000 0b2b3880-5936-4365-a048-eb17d2e5a6bf
fixed the error message when a pdf is open

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@5001 0b2b3880-5936-4365-a048-eb17d2e5a6bf