'Ensure that the course has been exported and return course export dir.'

def _get_course_export_dir(self):

    self._ensure_exported()
    block_path = os.path.join(self.root_export_dir, self.export_dir)
    self.assertTrue(os.path.isdir(block_path), msg='{}    is    not    a    dir.'.format(block_path))
    return block_path
