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

infra/perf-monitor/migrations/0003_add_site.sql

@@ -0,0 +1,7 @@
+-- Tag each measurement with the demo site it came from. Existing rows all
+-- belong to the baseline blog-demo; the cache-demo site was added later.
+
+ALTER TABLE perf_results ADD COLUMN site TEXT NOT NULL DEFAULT 'blog';
+
+CREATE INDEX IF NOT EXISTS idx_perf_site_ts ON perf_results(site, timestamp);
+CREATE INDEX IF NOT EXISTS idx_perf_site_route_region_ts ON perf_results(site, route, region, timestamp);