Prevent age-restricted fanbox posts from raising errors when source data
is fetched. This prevents error messages from being shown to users when
switching to the edit tab on a post.
This will cause uploads of age-restricted posts to fail with an
unrelated error because we either can't find the image url (if we were
given only the html page) or we can't download the image (because we're
not logged in to Fanbox).
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