nvmetcli: Report save name correctly

nvmetcli: Report save name correctly

When a user simply does 'nvmetcli restore' without
specifying a file name the default is used.  However, if the
restore fails you end up with the error message:

Error processing config file at None, error [Errno 1] Operation not
permitted: '/sys/kernel/config/nvmet/ports/0/ana_groups/1', exiting

Correct file name if None in error path.

Error processing config file at /etc/nvmet/config.json, error \
[Errno 1] Operation not permitted: \
'/sys/kernel/config/nvmet/ports/1/ana_groups/1', exiting

Signed-off-by: Tony Asleson <tasleson@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>