refactor(core): Improve top-level key validation in task runner (#16882)

packages/@n8n/task-runner/src/js-task-runner/errors/reserved-key-not-found.error.ts

@@ -0,0 +1,11 @@
+import { ValidationError } from './validation-error';
+
+export class ReservedKeyFoundError extends ValidationError {
+	constructor(reservedKey: string, itemIndex: number) {
+		super({
+			message: 'Invalid output format',
+			description: `An output item contains the reserved key <code>${reservedKey}</code>. To get around this, please wrap each item in an object, under a key called <code>json</code>. <a href="https://docs.n8n.io/data/data-structure/#data-structure" target="_blank">Example</a>`,
+			itemIndex,
+		});
+	}
+}