Oops, my inbox was getting full of uncaught promise rejections of `loadImage`!
I'm pretty sure they're caused when multiple images in a movie fail to load (e.g. network problems), but we fail to cancel them. So, the first failure would be caught as a part of `Promise.all` in `loadMovieLibrary`, but then subsequent failures wouldn't be caught by anything, and would propagate up to the console and to Sentry as uncaught errors.
In this change, we make a number of improvements to cancellation. The most relevant change for this bug is that `loadMovieLibrary` will now automatically cancel all resource promises when it throws an error! But this improved robustness also enabled us to finally offer a simple `cancel()` method on movie library promises, which we now available ourselves of at call sites, too.