A photo whose original is missing, does not decode or declares more than
4096x4096 pixels made attach.File.Thumb return an error, and every listing
that shows the photo answered 500 from then on: one 100-byte PNG uploaded
by any household member broke GET collections and the album for everyone.
Thumb now follows WinterCMS's File::makeThumb catch branch: it logs the
reason at warn level, stores WinterCMS's BrokenImage picture (exported as
attach.BrokenImagePNG) under the thumbnail key and returns its URL. Invalid
arguments, storage errors and encode failures are still errors.
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