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

demos/cloudflare/plugins/sandbox-test/manifest.json

@@ -0,0 +1,13 @@
+{
+	"id": "sandbox-test",
+	"version": "1.0.0",
+	"capabilities": ["read:content"],
+	"allowedHosts": [],
+	"storage": {
+		"logs": {
+			"indexes": ["timestamp"]
+		}
+	},
+	"hooks": ["content:afterSave"],
+	"routes": ["test", "logs", "restriction-test"]
+}