Lua lfs.attributes: Provide a more consistent error return

[cem]

Summary

In remaining error case, return 3-tuple consistent with the other error return
case.

Document how to invoke lfs.attributes() and detect/decode error return in
example comments.

[kevans]

+1 for consistency; it also feels like we should expose errno constants somewhere to lua, because that doesn't seem to be a thing at the moment.

[cem]

In D14451#302892, @kevans wrote:

+1 for consistency; it also feels like we should expose errno constants somewhere to lua, because that doesn't seem to be a thing at the moment.

+1