mount.glusterfs: return an error when mounting failed

When mounting fails, mount.glusterfs incorrectly returns 0 for some
error cases, it should return 1 instead. Also make sure that error
messages are redirected to /dev/stderr and not printed to stdout.

Unfortunately it is not possible with the current test-scripts to test
commands like 'mount -t glusterfs ...'. Any mounting of Gluster volumes
is done directly with the 'glusterfs' command instead.

Change-Id: Ica9d45b6d5ae537de869a1fa0f6c3edab47225d1
BUG: 1031973
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/7441
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>