More informative error message if vl-convert does not support the Vega-Lite version

[binste]

I noticed this when updating Altair to version 5 in an environment but forgot to update vl-convert and still used 0.8.0. When saving a chart I got RuntimeError: Unsupported Vega-Lite version string v5_8. Could this be improved so that it tells the user to either try upgrading or downgrading vlc depending on if the required version of Vega-Lite is higher or lower then the currently supported one by vl-convert?

RuntimeError: Vega-Lite version 5.8 is not supported. You might need to upgrade vl-convert <- same with downgrade, maybe is no longer supported. You need to downgrade vl-convert.

[jonmmease]

Yeah, that sounds reasonable! Thanks for the suggestion