context save: fails after deleting orphans, leaving CoreData in inconsistent state

[Mike McCracken]

in U1FilesClient fetchFolderWithResourcePath:... the last context save: can fail if an orphan to be deleted had an associated u1asset.

This is not handled, and so leaves the object graph in a bad state.

The problem is in the data model:

Asset and FileNode are related. A FileNode can have an optional Asset, but an Asset must have a related FileNode.
However, the FileNode->Asset relationship delete rule is just 'Nullify', meaning that when we delete a fileNode we do not delete its related asset, we just leave it sitting around with a nil fileNode relationship, making it invalid, and causing errors when we save.

The solution is to make the delete rule "Cascade".