Change to error404

I mentioned the new error404 feature in the last entry - after some review, I decided to change this a bit. There was no reason for it to be at the Resolver level, setting at Publisher made more sense. So I changed things around to reflect this new design. However, if you previously subclassed Publisher to override the _error404 method, that feature is now broken. You'll have to rewrite your custom _error404, which should be trivial.