When s3d still lists a file as waiting but its buffered copy is gone,
Upload now fails with a raw admin error and nothing says which files are
behind it. Overview and Drives now list them (waiting, copy missing in
two checks a moment apart, no upload running, older than two minutes),
each with Upload again… (replace it from the original) or Remove, and
Upload now explains the cause instead of passing s3d's error through.
