'Tests for missing child nodes.'

def test_missing_child_nodes(self):

    graph = MigrationGraph()
    graph.add_node(('app_a', '0001'), None)
    msg = "Migration    app_a.0002    dependencies    reference    nonexistent    child    node    ('app_a',    '0002')"
    with self.assertRaisesMessage(NodeNotFoundError, msg):
        graph.add_dependency('app_a.0002', ('app_a', '0002'), ('app_a', '0001'))
