Previously, it just silently didn't do anything which is not very
intuitive. Since the lua api returns an error string, check to see if
it matches the "property not found" case and print an error message.
Additionally, don't add the fake property to the internal
cached_properties list or try to observe it. This avoids redundant
evaluate calls which will never actually succeed. We do still mark it
under watched_properties however. This avoids having to call
mp.get_property_native multiple times.