'Test stored_context error path.'

def test_stored_context_err(self):

    self.stack = stack.Stack(self.ctx, 'creds_stack', self.tmpl)
    ex = self.assertRaises(exception.Error, self.stack.stored_context)
    expected_err = 'Attempt    to    use    stored_context    with    no    user_creds'
    self.assertEqual(expected_err, six.text_type(ex))
