Clarify notification worker app dependency

scripts/notification-worker.mjs

@@ -55,6 +55,17 @@ async function callNotificationEndpoint(path) {
   return body;
 }
 
+function formatWorkerError(error) {
+  if (error?.cause?.code === "ECONNREFUSED") {
+    return [
+      `wtr. notification worker: app is not reachable at ${appUrl}.`,
+      "Start the Next.js app first with `npm run start`, or set WTR_APP_URL to the running app URL.",
+    ].join(" ");
+  }
+  if (error instanceof Error) return error.message;
+  return String(error);
+}
+
 async function tick() {
   if (isRunningTick) return;
   isRunningTick = true;
@@ -72,7 +83,7 @@ async function tick() {
       lastMorningBriefDate = warsawTime.dateKey;
     }
   } catch (error) {
-    console.error(error);
+    console.error(formatWorkerError(error));
   } finally {
     isRunningTick = false;
   }