mount: fix --all and nofail return code

Now the "nofail" affects warnings warning messages only. That's wrong
and regression (against original non-libmount version). The nofail has
to control return code too.

Reported-by: Patrick McLean <chutzpah@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>
umount: more robust success message for --all

Signed-off-by: Karel Zak <kzak@redhat.com>
mount: apply "nofail" to MNT_ERR_NOSOURCE libmount error

 fstab:
     UUID=nonexist /mnt/nonexist1 ext4 nofail 0 1

 # mount -av
 mount: can't find UUID=nonexist

.. this is bug of course.

Reported-by: Patrick McLean <chutzpah@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>