/** Validate the value for the property. Answer null if valid.
	* Answer the error String if invalid.
	* Override this method to modify the validation behavior.
	* Default is just to use the validator from _getValueValidator.
	* You may change ValueValidator to override the error messages
	* or the generic behavior inherited from PntValueValidator
	* @param prop PntPropertyDescriptor
	* @param value mixed value to be validated.
	* @return String the error string or null if the value is valid
	*/
	function validateGetErrorString($prop, $value, $validateReadOnly=true) {
		$validator = $this->getValueValidator($prop);
		return $validator->validate($value, $validateReadOnly);
	}