nice -19 tar cvfz /backup/mybackup.tar.gz ./directory_under >result &

has the merit to tell me what was the last file that was tar before an
error took place.
However writing all those names in a file consume time, especially when
you have 30 calls to tar in a script.
Is there an elegant way to get only the error message in the output
file (when there is) + the last file which was successfully tar?