netinfo: Error out if we failed to change a bond's mode

This is currently caused by NetworkManager (even with a 'server'
configuration). Please see BZ 1136843.

Also check for the failure in a unit test.

Change-Id: I6c29bd5aee13cd6f0be1ab7f308b5432f264215c
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32657
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>