Fix back_up_files() error detection when opening/CRCing the file

            FileState& g = newSnapshot.editValueAt(m);

            int fd = open(realFilename.string(), O_RDONLY);

            if (fd != -1) {

            if (fd < 0) {

                // We can't open the file.  Don't report it as a delete either.  Let the

                // server keep the old version.  Maybe they'll be able to deal with it

                // on restore.

                LOGP("Unable to open file %s - skipping", realFilename.string());

            } else {

                g.crc32 = compute_crc32(fd);