umount: consider helper return status for success message

umount: consider helper return status for success message

If a helper function was executed to unmount, we simply return
without any user feedback. That can unintentionally surpress
verbose messages (`--verbose`) for non-root users who use udisks2
to mount filesystems, and unmount via the unmount.udisks2 helper.

It would be better to check the helper return status as well for
completeness and a more reliable way to test the success of the
unmount operation.

mnt_context_get_helper_status() is only called if the helper was
executed, i.e. mnt_context_helper_executed == 1, anything else
wouldnt make sense anyways.

Addresses: #3790
Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>