GetPermissionTarget returns non-nil err when permission target doesn't exist

[M0roSan]

GetPermissionTarget returns non-nil err when permission target doesn't exist. I would like it to return nil err when permission target doesn't exist.

This will be useful when I perform Create/Update/Delete permission target operation. I initially check whether the permission target exists in the system.

Other instances

• GetUser returns nil err when a user doesn't exist
• GetGroup returns nil err when a group doesn't exist

[SteveNewson]

I think the above PR addressed this issue, so it can probably be closed.