Fix (+test) phantom node onion error returning

[TheBlueMatt]

See #1199 (comment) and the following comment, probably need to test al the cases where we return onion errors in that function. Also some nits worth hitting at #1199 (review)

[jkczyz]

Also want to tag module-level docs for follow up, but wouldn't mind leaving for a more general documentation clean-up. #1199 (review)