Good question 
If the JSON is in unexpanded input form, a tooltip may appear below with an error message. However, if the input is expanded (editor), I believe there is nothing (apart JSON correction with Monaco).
Validation is done on input, switching to an editor does not trigger validation until the editor is closed.
I don't think we can extend validation to the editor 
returns String to show an error message
Note that the current version of NR may prevent you from seeing the tooltip
See the FR and actual work here
