libmount: small cleanup in umount code

Signed-off-by: Karel Zak <kzak@redhat.com>
umount: provide more helpful error message

non-root user on device that is not mounted:

 $ umount /mnt/flash
 umount: /mnt/flash: umount failed: Operation not permitted

this is true, because we cannot evaluate permissions as there is no
entry in mtab, but users expect something more helpful -- for example

 umount: /mnt/flash: not mounted

References: https://bugs.archlinux.org/task/31647
Signed-off-by: Karel Zak <kzak@redhat.com>