[Seaside] How to signal 404 early?

[Todd Blanchard]

Like when I've got the request in the start: aRequest method of my 
WARenderLoopMain subclass.

Specifically, Smallblog looks at the url in the request in its start: 
aRequest method and does some different things based on the path 
components.

One of the things it does is assume anything after the /blog/ part of 
the path that isn't 'admin' must be the name of a blog.  Of course, the 
blog retrieval machinery will create a new blog for a given name if it 
can't find one - so mistyped urls result in automatic generation of new 
blogs.

I'd prefer to simply abort the request handling at this point and cough 
up a 404, but I don't seem to have access to the response at this 
point.