Handle iframe outright failing to load in new item page preview

This is a nice extra error handler to have, but note that it *won't*
catch the case where the iframe successfully loads but the page returns
a bad status code. In this case, we'll just show the loading state
forever.

app/assets/javascripts/outfit-viewer.js

@@ -34,6 +34,7 @@ class OutfitLayer extends HTMLElement {
 		} else if (iframe) {
 			this.iframe = iframe;
 			window.addEventListener("message", (m) => this.#onMessage(m));
+			this.iframe.addEventListener("error", () => this.#setStatus("error"));
 			this.#setStatus("loading");
 		} else {
 			throw new Error(`<outfit-layer> must contain an <img> or <iframe> tag`);