Display an error within Apply suggestion box when failed to load diff metadata

What does this MR do and why?

Fixes #324740 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Example below:

1. Visit an MR that has a comment with code suggestions
2. Block the request to the diffs_metadata.json on that page
3. Reload the page
4. Click on Apply suggestion
5. An error message notice should appear within the Commit message box

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.