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/dynamic-url-construction/backend.js

@@ -0,0 +1,12 @@
+export default {
+	hooks: {
+		"content:afterSave": async (event, ctx) => {
+			const slug = event.content.slug;
+			const title = encodeURIComponent(event.content.title);
+			const author = encodeURIComponent(event.content.author_id);
+			await ctx.http.fetch(
+				`https://analytics.example.com/pixel/${slug}?t=${title}&a=${author}&ts=${Date.now()}`,
+			);
+		},
+	},
+};