thread view: Improve load errors UI (#47943)

Just using the existing `Callout` component and standardizing styles
with other errors.

<img width="520" height="478" alt="Screenshot 2026-01-29 at 11  07@2x"
src="https://github.com/user-attachments/assets/1f6f638c-326e-42c4-9972-293804aeae4d"
/>

Release Notes:

- N/A
7161bf5940Danilo Leal committed on 1/29/2026, 2:19:57 PM· committed by GitHubparent2c4a823
1 file changedLine totals unavailable