We should probably have a try/catch in MessageListView itself
too, for post-processing kind of stuff, but we want to make
this new module defensive in its own right.
Having submessages will become common enough that the
info message here is too spammy, and there are other
ways to observe incoming submessages if you're doing dev
debugging, which this was originally written for.
This commit lays the foundation to handle submessages for
plugin widgets. Right now it just logs events, but subsequent
commits will add widget functionality.
