Move `error` from top_stmts and top_stmt to stmt

By this change, syntax error is recovered smaller units.
In the case below, "DEFN :bar" is same level with "CLASS :Foo"
now.

```
module Z
  class Foo
    foo.
  end

  def bar
  end
end
```

[Feature #19013]

1 files changed, 2 insertions, 0 deletions

diff --git a/ast.c b/ast.c
index 67275c47b3..c9d0b41fac 100644
--- a/ast.c
+++ b/ast.c
@@ -648,6 +648,8 @@ node_children(rb_ast_t *ast, const NODE *node)
                                         NEW_CHILD(ast, node->nd_pkwargs),
                                         kwrest);
         }
+      case NODE_ERROR:
+        return rb_ary_new_from_node_args(ast, 0);
       case NODE_ARGS_AUX:
       case NODE_LAST:
         break;