vdev_id: Add error message when $CONFIG is missing

It was observed that vdev_id exists silently when
the $CONFIG file is missing.

This patch adds error message in case vdev_id is
called without default $CONFIG or '-c'. This makes
end user observe the exit message more easily.

Before Patch:
~~~~~~~~~~~~~
$ ./cmd/vdev_id/vdev_id
$

After Patch:
~~~~~~~~~~~~
$ ./cmd/vdev_id/vdev_id
Error: Config file "/etc/zfs/vdev_id.conf" not found
$

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Closes #11498