A decode failure in the face sweep was counted, logged at debug where
nobody saw it, and left unmarked — so the next pass fetched the same
file and failed the same way. For the 521 MB panorama behind rawler's
panic that was half a gigabyte per sweep, on a tablet. It is now marked
examined with nothing found and a zero edge, which is what a later "try
again with a better decoder" pass would select on, and the warning
names the file. The failure count is unchanged: it did fail.