assistant: Show an error when `/fetch` returns no content (#16001)

This PR makes it so the `/fetch` slash command will display an error
when the URL does not have any textual content:

<img width="308" alt="Screenshot 2024-08-08 at 4 51 55 PM"
src="https://github.com/user-attachments/assets/bf4cbc18-c65b-48a2-aedd-26b50f47f018">

Release Notes:

- N/A