Checks all parameters for invalidating conditions

        :returns: str -- message if error, 0 otherwise

def verify(self):
        """Checks all parameters for invalidating conditions

        :returns: str -- message if error, 0 otherwise
        """
        for row in range(self.nrows()):
            result = self.verify_row(row)
            if result != 0:
                return result
        return 0