Из класса Кохана Validation
, errors
исходный код метода:
* Returns the error messages. If no file is specified, the error message
* will be the name of the rule that failed. When a file is specified, the
* message will be loaded from "field/rule", or if no rule-specific message
* exists, "field/default" will be used. If neither is set, the returned
* message will be "file/field/rule".
*
* By default all messages are translated using the default language.
* A string can be used as the second parameter to specified the language
* that the message was written in.
*
* // Get errors from messages/forms/login.php
* $errors = $Validation->errors('forms/login');
Попробуйте использовать $errors = $post->errors('models/user')
вместо $errors = $post->errors('signup')