Move existing customError. Add more to topic: webapi, feature: customError

[renoirb]

URL: https://docs.webplatform.org/wiki/dom/ValidityState/customError

If you look at current compat data we get some data at topic: webapi, feature: htmlelement associated with a sub feature details formNoValidate, validationMessage, validity, willValidate properties and setCustomValidity() and checkValidity() methods.