marsadm: better error message at failed 'invalidate'

userspace/marsadm

@@ -1464,6 +1464,7 @@ sub invalidate_res_phase3 {
   my ($cmd, $res) = @_;
   my $dst = "$mars/resource-$res/syncstatus-$host";
   my $primary = _get_designated_primary($res);
+  ldie "Cannot execute 'invalidate' because noone is designated as primary.\n" if (!$primary || $primary eq "(none)");
   my $replay = get_link("$mars/resource-$res/replay-$primary");
   $replay =~ m/^log-([0-9]+)-/ or ldie "replay link '$replay' is not parsable\n";
   my $replay_nr = $1;