Server Side Validation - Allow Message to Come From Database

Server side validation can only be done using a single language user message statement which has to be built into the server action file.

This is a feature requeest to allow the message in server connect to come from a variable value by adding the lightning icon functionality here:

EDIT - I was also struggling to get a server side validation error to trigger these two client side values:
.invalid
validationMessage
See the discussion below for details of how to do this!