Try different validator

[domoritz]

From https://github.com/vega/vega-editor/issues/191

We would like

• Better error messages
• Links to the editor from an error

[nyurik]

Would be awesome if the editor also supported hjson - much better for writing it - allows comments, not as strict as json, etc.

[domoritz]

@nyurik We had this discussion many times. I only want to support more languages if we don't cause confusion with the docs(excludes yaml) and supports autocomplete in the editor (which excludes hjson, I think). But I agree that JSON is a terrible language for people to write.

[nyurik]

I guess I might have to invest into hjson schema validator and autocomplete module ... will see :)

[domoritz]

It would be good if we could use validator with better error messages. Right now, we use AJV and the error messages are not very helpful in most cases (#643)

[domoritz]

Maybe a simple option would be to just read the error messages from Monaco and get rid of ajv.