Global Styles: Additional CSS may not display any error on failure to save #74447

Description

The global styles "additional CSS" feature may fail to save without displaying any information to the user.

This happens at /wp-admin/site-editor.php?p=%2Fstyles&section=%2Fcss:

If you click on the page to open the full block editor canvas, the error messages appear:

Any errors should be displayed in both views.

Related to #57245.

Step-by-step reproduction instructions

1. Visit /wp-admin/site-editor.php?p=%2Fstyles&section=%2Fcss.
2. Enter /* </style> */ in the additional CSS input.
3. Open the network tab in the browser console to observe the failed save request.
4. Save the changes.
5. Notice that no error message is displayed.
6. Notice that the browser responds with a 400, the content has not been saved.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure