Fix EZP-22793: Incorrect kernel error when accessing hidden node.

JIRA: https://jira.ez.no/browse/EZP-22793

Because access to hidden nodes is not controlled by user permissions or limitations (but rather `ShowHiddenNodes` ini setting ), the generated error should not be `KERNEL_ACCESS_DENIED`.

This PR modifies it to `KERNEL_NOT_AVAILABLE` instead (which is kernel error 3, or HTTP Error 404 by default)