PropagateDownload: Throw an error if the file is empty while it should not have been...

PropagateDownload: Throw an error if the file is empty while it should not have been (#4753)

If the downloaded file is empty but the PROPFIND previously announced it
should not have been empty, this might mean the file was somehow corrupted
because of a bug on the server and that we should therefore not accept
the file.

Normaly we accept a change between the actual size of the file and what we
got during discovery because the file might have been updated to a new version
inbetween. But after this patch we won't accept the file if it was replaced
by an empty file.

Will help for issue #4583
Also requested by IL for issue 548