Today I was reading Brad Abrams's blog and I've noticed that he was talking about a book called Microsoft Ajax Library Essentials. At the end Brad says that "(...) I think the book misses out on talking about how to use the Ajax Control Toolkit completely from client side javascript, but maybe that is for the 2nd edition ;-)".
I think that I can answer that. The last time I looked (and I know it was really a long time ago), that wasn't really possible. Why? simply because at the time the client error messages were maintained as embedded resources. Unlike what happens with the AJAX library, they didn't have any download for the js files which means that you'd have to 1.) use the ScriptManager control to get those errors injected on the page or 2.) write some code that would go through the resources and export it to a js file which could be embedded as a resource.
Since several months have passed since then, I'm hoping that I'm wrong on this...though I wouldn't bet on that.