Added more info to the exception
Generating conflict for deleting a modified node
Dealing with delete conflicts in the update
Deletion now keeps the node in the manager but marks it as delete

This allows me to deal with deletes in a consistent way.
Throwing a conflict when someone tries to modify a deleted node

This should only happen during merging. Hopefully. We'll see...
Sanity test for the Updates

Again, making sure you can't change a deleted node