Handle API save errors when saving user's file-by-file preference from the MR page
The following discussion from !55931 should be addressed:
- [ ] @thomasrandolph started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55931#note_545261241):
> We're going to handle the errors for this save separately.
>
> A number of reasons:
>
> - Showing errors in a UI that's **already** a popover is complex
> - Errors here are very unlikely since it's a low-traffic endpoint
> - Failures on save are non-catastrophic. Worst case scenario is a different device has an out-of-sync setting.
issue