THE ANSWER

Well as it happens many times in my life – i found the answer myself.
For people who struggles as me here the answer:

$post['errors']['field_name']['errors'][] = __('Error text here.');

if you do that – error text will be displayed at the bottom of that field. In my case this doesn’t work because i’m using inline media uploader, but with standard WP media uploader (the popup version) it’s working just fine.

BTW best article about custom fields is here: http://net.tutsplus.com/tutorials/wordpress/creating-custom-fields-for-attachments-in-wordpress/