The default validation error messages are imported in your app's validators folder.
If you want to change or extend them, all you need to do is create a messages.js file under app/validators.
// app/validators/messages.js
import Messages from 'ember-cp-validations/validators/messages';
export default Messages.extend({
uniqueUsername: '{description} {username} already exists'
});
Within this object, you can overwrite the default messages or create new messages just like in the example above.
If a message of a given type is not found, it will default to the invalid message.
Usage examples can be found here