Expose classes of exceptions with webservice_error() instead of calling export() on each instance.

[Leonard Richardson]

There are a few places in Launchpad where we call lazr.restful.error.expose() on exception instances to make them yield a 400 response code instead of an OOPS. This was necessary because the mechanisms for associating response codes with entire classes of exception didn't work. I've fixed those mechanisms in lazr.restful and we're ready to get rid of the expose() hacks.