msglist: When we fail to load the initial batch of messages, don't show infinite loading spinner

When we open a message list, we try to fetch at least a screenful or so of messages, and then load more as the user scrolls up or down.

That initial fetch can fail, like any request, and in that case we currently just show an infinite loading spinner. We should do better: maybe show the error message we received from the server, if any, and a way to ask the app to retry the fetch.