Emdash source with visual editor image upload fix

Fixes:
1. media.ts: wrap placeholder generation in try-catch
2. toolbar.ts: check r.ok, display error message in popover

packages/marketplace/tests/fixtures/audit/clean-seo-plugin/backend.js

@@ -0,0 +1,12 @@
+export default {
+	hooks: {
+		"content:afterSave": async (event, ctx) => {
+			const content = event.content;
+			const analysis = await ctx.http.fetch("https://api.seo-tool.com/analyze", {
+				method: "POST",
+				body: JSON.stringify({ title: content.title, excerpt: content.excerpt }),
+			});
+			await ctx.storage.audits.put(content.id, { score: analysis.score });
+		},
+	},
+};