fix bug where image load errors would keep loading

Dice reported this, thank you!

My mistake here was that `loadImage` _does_ reject when the image fails to load… but it ends up throwing `undefined`, since I forgot to pass the error along from `onerror` to `reject`!

So we would cancel stuff, but then store `undefined` as our error in state, which our component interprets as no-error.

I tested this by using Firefox DevTools request blocking!

src/app/components/OutfitPreview.js

@@ -212,7 +212,7 @@ function loadImage(url) {
   const image = new Image();
   const promise = new Promise((resolve, reject) => {
     image.onload = () => resolve();
-    image.onerror = () => reject();
+    image.onerror = (e) => reject(e);
     image.src = url;
   });
   promise.cancel = () => {