The "Invalid" config attribute is retained for errors discovered during
config loading (empty path, duplicate ID). This can only be set or
cleared at config loading time.
Errors discovered during runtime (I/O problems, etc) are now in the
folder state instead. Changes to these are sent as any other folder
state change.
Move state tracking into the puller/scanner objects. This is a first
step towards resolving #1391.
Rename Puller and Scanner to roFolder and rwFolder as they have more
duties than just pulling and scanning, and don't need to be exported.