Changes error on Language from base to certain attributes

1) If one tried to unpublish the default language, the error message was not shown in the form. By setting the error object to the public attribute will make the error notice visible for the user.

Fixes #487 

2) If one unchecked the default checkbox of the default language, the page could not be saved, which is absolutely correct. But there was missing a notice for the user. By setting the error object to the default attribute, the notice is now shown for the user in the form.