Improve pre_transform_values error message

[jonmmease]

When a chart spec includes a Vega transform or signals that are not supported, the pre_transform_values function raises an error message like this:

ValueError: Pre-transform error: Requested variable (Variable { name: "data_2", namespace: Data }, [])
 requires transforms or signal expressions that are not yet supported

It would be much more helpful if the error included a description of exactly what feature is not supported. This will require some significant refactoring to the planner, but should be doable.