'Tests for forwards/backwards_plan of nonexistent node.'

def test_plan_invalid_node(self):

    graph = MigrationGraph()
    message = "Node    ('app_b',    '0001')    not    a    valid    node"
    with self.assertRaisesMessage(NodeNotFoundError, message):
        graph.forwards_plan(('app_b', '0001'))
    with self.assertRaisesMessage(NodeNotFoundError, message):
        graph.backwards_plan(('app_b', '0001'))
