ceph-disk: Reporting /sys directory in get_partition_dev()

ceph-disk: Reporting /sys directory in get_partition_dev()

When get_partition_dev() fails, it reports the following message :
    ceph_disk.main.Error: Error: partition 2 for /dev/sdb does not appear to exist
The code search for a directory inside the /sys/block/get_dev_name(os.path.realpath(dev)).

The issue here is the error message doesn't report that path when failing while it might be involved in.

This patch is about reporting where the code was looking at when trying to estimate if the partition was available.

Signed-off-by: Erwan Velu <erwan@redhat.com>