Outbox error handler docs

> the personal inbox delivery.

[shared inbox delivery]: https://www.w3.org/TR/activitypub/#shared-inbox-delivery

Error handling

--------------

Since an outgoing activity is not immediately processed, but enqueued to the

queue, the `~Context.sendActivity()` method does not throw an error even if

the delivery fails.  Instead, the delivery failure is reported to the queue

and retried later.

If you want to handle the delivery failure, you can register an error handler

to the queue:

~~~~ typescript{6-9}

import { Federation, InProcessMessageQueue } from "@fedify/fedify";

const federation = new Federation({

  // Omitted for brevity; see the related section for details.

  queue: new InProcessMessageQueue(),

  onOutboxError: (error, activity) => {

    console.error("Failed to deliver an activity:", error);

    console.error("Activity:", activity);

  },

});

~~~~

> [!NOTE]

> The `onOutboxError` callback can be called multiple times for the same

> activity, because the delivery is retried according to the backoff schedule

> until it succeeds or reaches the maximum retry count.

  /**

   * A callback that handles errors during outbox processing.  Note that this

   * callback can be called multiple times for the same error, because failed

   * deliveries are retried.

   * callback can be called multiple times for the same activity, because

   * the delivery is retried according to the backoff schedule until it

   * succeeds or reaches the maximum retry count.

   *

   * If any errors are thrown in this callback, they are ignored.

   *