Bug/import

Publish was breaking if the were children references to non-existent items. Because import uses publish, this was breaking imports of some courses.

Import was breaking if a tar.gz had a directory in it w/ the same name as the root dir which occurred importing some MITx courses with manifest files.

@ormsbee @cpennington plz review
The docstring says that ItemNotFoundError is raised if any subtree nodes aren't found. This changes that behavior, right?