in the given example :

public function setDefaultOptions(OptionsResolverInterface $resolver)
{
    $resolver->setDefaults(array(
        'error_mapping' => array(
            'matchingCityAndZipCode' => 'city',
        ),
    ));
}

in this reference :

The left side of the error mapping also accepts a dot ., which refers to the field itself. That means that any error added to the field is added to the given nested field instead;

"left side" is actually the wrong side, I mean the "right side"...