[TYPO3-mvc] validate domain object in own validator

[Domi]

OK found the right solution, I need still to use the deprecated 
$this->errors array:

$this->errors['email'] = new Tx_Extbase_Validation_PropertyError('email');

$this->errors['email']->addErrors(array(new
Tx_Extbase_Validation_Error('User with this email already exists', 
99988776)));

Cheers Dominic