Task creation should fail if spawner doesn't have property

[phil-brad]

If you have unknown properties in the spawner, these simply get added as an attribute of the spawner object (unless the spawner overloads __setattr__). Following code from generate_tasks_from_spec:

    if node.has_property:
        value = _check_type(task_spawner, node.property_name, node.property_value)
        if isinstance(node, IndexedNode):
            array = getattr(task_spawner, node.property_name)
            array[node.index] = value
        else:
            setattr(task_spawner, node.property_name, value)

In the else statement, perhaps a hasattr check should be done first.