Relaxed handling of problems while copying attributes.

Mono fires Exception "Invalid parameter" instead of ArgumentOutOfRangeException if DateTime in SetLastWriteTimeUtc() is out of range.
Copying date and time of hard linked files is debatable.
So we now silently accept these kinds of problems without error messages.
searching for old backups means searching for directories...
while copying check hashes of old backups against checksum files --> warning if there was a corrupt file
warning msg