LookupError sounds good.  About the latest patch: I wonder if using !a instead of !r in the format strings for exceptions would be more helpful (maybe you’ve seen a recent python-dev subthread about that).  I don’t like seeing escapes for perfectly common characters like ß or é, but OTOH escapes help disambiguating different characters that look the same.  Apart from these two points, this is good to go.

A doc addition for LookupError would be an independent changeset; it has nothing to do with adding shutil.chown.