Data integrity: Internet Archive ID is 't' generating bad thumbnails in carousels

This relates to the slack conversation between @mekarpeles and Brittany. For those who missed it, apparently this fix is not applying to:
https://openlibrary.org/subjects/in_library#ebooks=true&sort=edition_count
when you click right arrow.

The carousel item is rendered on the server side so this is likely an issue in the template.

The template uses the following code
If an Internet Archive ID is available that's used here.
```
$elif book.get('ia'):
$ cover_url = '%s/b/ia/%s-M.jpg?default=%s'%(cover_host, book.get('ia')[0], fallback_cover)
```

For some reason some books have an inaccurate `ia` field. When that happens a bad URI is created like //covers.openlibrary.org/b/ia/t-M.jpg?default=https://openlibrary.org/images/icons/avatar_book.png

The template is dumb and will assume if it finds an image a cover exists. ie. It;s the default parameter in the covers api that is causing an issue.

The covers api takes a default parameter.

Two solutions to this
1) Either we cannot depend on the IA field and we should not ever use it for the thumbnail
2) API should be updated to 404 when an image is not available so alt text can be used.

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Borrow

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### Evidence / Screenshot (if possible)

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### Related

Very similar to #2836 possibly can be fixed at the same time.

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