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.
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