Before this change, koofr failed certain bisync tests because it can't set mod
time without deleting and re-uploading. This caused the "nothing to transfer" log
to not get printed where expected (as it is only printed when there are 0
transfers, but koofr requires extra transfers to set modtime.)
This change fixes the issue by ignoring the absence of the "nothing to transfer"
log line on backends that return `fs.ErrorCantSetModTimeWithoutDelete` for
`obj.SetModTime`.
nielash
26b629f42f