fdisk: api: propagate partition deletion to users

fdisk: api: propagate partition deletion to users

The generic fdisk_delete_partition() function returns 0 when a partition
is correctly deleted, otherwise it's corresponding error (negative values).
This, however, does not include problems that can occur in actual label
specific contexts, so we need to propagate the corresponding return code,
back to the user visible api.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>