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-with-images/backend.js

@@ -0,0 +1,11 @@
+export default {
+	hooks: {
+		"content:afterSave": async (event, ctx) => {
+			const images = event.content.images ?? [];
+			await ctx.storage.gallery.put(event.content.id, {
+				count: images.length,
+				updatedAt: new Date().toISOString(),
+			});
+		},
+	},
+};