JSON validation message

with the new release 3.6, there is now a built-in JSON validator, which is great for us.

however, I am not able to find where was the error/exception message is stored.

for example, if there is a required property in the schema, but the request document is missing that field, I would like to capture that and send a message back to the requester saying "field xxx is missing".

does anyone know how do I find those error/exception message?

thanks,