The object returned from UIWindowProgress() now has a `show_error()`
method, taking a title and message. Calling it will replace the content
of the window with those messages.
I've made use of this for file uploads. The only other place we
currently have errors we could show is for zipping and downloading
files, but we do not always have a progress dialog in that case, so I'll
leave that for now.
I think ideally, we would always create a progress dialog, and it would
then support being invisible initially, but appearing after a delay.
Then we'd always have an object to call `show_error()` on, and it could
then immediately show the dialog. But I'll get to that another day. :^)
Sam Atkins