man-page xref: make nested-structure warnings fatal

Followup to:
  - #21060, where I added new struct checks (but did not make them fatal)

  - #21534, which added per-interface stats and a .Network field,
    but its documentation was slightly off

Signed-off-by: Ed Santiago <santiago@redhat.com>

hack/xref-helpmsgs-manpages

@@ -256,6 +256,7 @@ sub xref_by_help {
                     }
 
                     warn "$ME: 'podman @subcommand {{$k' $msg\n";
+                    ++$Errs;
                 }
             }
         }