Allow resolving to roots that have been unlinked

Don't make the realpath error a blocker for removing stale entries.

These should happen any more (since
76f6d5f17e68ad01d8c7a8901e5070191a177cd6) but just in case we end
up with roots in our list that don't exist on the filesystem, allow
removing them even if they don't exist any more.
clarify the legacy pattern syntax a bit better