Download: Retrigger folder discovery on 404

Download: Retrigger folder discovery on 404

See owncloud/enterprise#1966

If the server and the client's database go out of sync, there could be
persistent 404 errors. This change ensures that the problem corrects
itself eventually by triggering a remote discovery of the file's
parent folders.

It does not address the root cause that might have lead to the
divergence.