[Vega] Improve the error when schema is messed up

[stratoula]

Describe the feature:
There are cases that the schema of the vega spec can be messed up:
Examples:

1. Mess up with the title
   

2. Add chars after the .json
   

This generates an error which is not so easy to understand. The fix is easy, ensure that your schema is correct. but we should ensure that the error is easier to understand. Something like:
Ensure that vega schema is set correctly or something like that. We can work on this with the @elastic/kibana-docs team

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[stratoula]

Btw vega online editor handles it correctly

[KOTungseth]

Would it make sense to include exactly what users should include? For example:

The URL for the JSON "$schema" is incorrect.
If you are using Vega, use https://vega.github.io/schema/vega/v5.json.
If you are using Vega-Lite, use https://vega.github.io/schema/vega-lite/v5.json.

If we don't want to include the schema strings, about how:

The URL for the JSON "$schema" is incorrect. Correct the URL, then click Update.

[stratoula]

Thanx Kaarina! I prefer the shorter version

The URL for the JSON "$schema" is incorrect. Correct the URL, then click Update.

I am not sure if we can identify the vega library used. If we can we could go with:

The URL for the JSON "$schema" is incorrect. Use `https://vega.github.io/schema/vega-lite/v5.json`, then click Update.