Assertion/Warning when setting action.data

[lepmik]

When using action.data['name'] = 'name.something' it is nicely retrieved with action.data_path('name'), however, if an absolute path or relative path is set in action.data, data_path will not work properly. Either we can assert that the 'name.something' is not a path or we can warn and say that data_path will not work, or just disabling it. I vote for an assertion, i.e. forcing the user to put data in 'action_path/data'