Get error messages from Zend_Form and response as json
Have you tried getMessages? I think this is the method you'd like to use to get human-friendly error messages.
You wrote you've tried getErrorMessages and getErrors, but getMessages is a different beast altogether, that's why I'm asking whether you've tried it.
getErrors returns codes, getErrorMessages returns registered custom error messages (seems probable you have none), while getMessages returns the actual human-friendly error messages.