https://bugs.musicpd.org/view.php?id=4593

Move code from config_parse_path().

Unused.

If an error has already been set by f.directory->storage->GetInfo(),
don't set it again.

https://bugs.musicpd.org/view.php?id=4577

It was used in a wrong way, which did not deal with errors
consistently.  And if that's wrong, there is no need for FindStorage()
at all - let's remove it and the confusion around it.