Comet UpdateDefaultXml message->UpdateDefaultHtml.

We’ve moved towards naming things “html” when dealing with
comets, so this follows that trend.
Add snippet failure type for missing comet type.
Rework Comet snippet.

In particular, the snippet now exposes a
Comet.containerForCometActor method that can be used by
an arbitrary third party consumer to produce the container for
a given comet actor when a handle to the LiftCometActor
object already exists.

It is, of course, still possible to materialize a CometActor using
regular lift:comet snippet.