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/expected.json

@@ -0,0 +1 @@
+{ "verdict": "pass", "maxRiskScore": 10, "categories": [], "images": "pass" }