Get error messages from Zend_Form and response as json

I'm trying to get error messages from Zend_Form and response as json. What is the best practice of getting Zend_Form errors and replying as json?

<?

class SomeController extends Zend_Controller_Action {

    public function indexAction() {

        $form = new Application_SomeForm();
        if ($form->isValid( $this->getRequest()->getPost() )) {
            //do something here
        }       
        $this->_helper->json($form->getErrorMessages());

    }

}

I can't get errors via $form->getErrorMessages(), but errors are present if tested print_r($form->gerErrors())

Array
(
    [email] => Array
        (
            [0] => isEmpty
        )

    [password] => Array
        (
            [0] => isEmpty
        )

    [foreign] => Array
        (
        )

    [login] => Array
        (
        )

)

So, my questions are:

a) How to get all error messages for form?

b) Is there any Json Wrapper for resposning ajax submitted forms? For example $jsonResponse->setErrorStatus()->addFormErrors($form)