Plotly list instantiation

Before this raised a confusing *validation* error:
```python
Data(Scatter(x=[1,2,3], y=[2,1,2]))
```

The reason being that if you unpack a dictionary into args (`*args`),
you get a list of *keys*, which is never going to make a valid Plotly
graph object. Therefore, we just catch it and raise a more helpful
*instantiation* error instead.
Ultimately, this is the error message that pops out. Because it's an instantiation error, we strictly *shouldn't* see this message anywhere else than manual user instantiation (i.e., not in validation, to_graph_objs, force_clean, etc.).

Also, this will never come up on the backend in django because we instantiate empty objects, add to them, then validate. So this functionality should remain uniquely to Python usage (i.e., there's no need to add a universal `plain_message` that other languages might use).