gnt-backup: Don't show confusing message w/o target node

“gnt-backup export” requires the target node. Until now, the master
daemon would complain that the “parameter
'OP_BACKUP_EXPORT.target_node' fails validation”. With this patch,
an additional check is done in the client program.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Manuel Franceschini <livewire@google.com>