Handle API save errors when saving user's file-by-file preference from the MR page

The following discussion from !55931 (merged) should be addressed:

• @thomasrandolph started a discussion:

  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.