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/node.h b/node.h
index d97c333ab5..2a27988ff5 100644
--- a/node.h
+++ b/node.h
@@ -125,6 +125,7 @@ enum node_type {
     NODE_ARYPTN,
     NODE_HSHPTN,
     NODE_FNDPTN,
+    NODE_ERROR,
     NODE_LAST
 };
 
@@ -386,6 +387,7 @@ typedef struct RNode {
 #define NEW_PREEXE(b,loc) NEW_SCOPE(b,loc)
 #define NEW_POSTEXE(b,loc) NEW_NODE(NODE_POSTEXE,0,b,0,loc)
 #define NEW_ATTRASGN(r,m,a,loc) NEW_NODE(NODE_ATTRASGN,r,m,a,loc)
+#define NEW_ERROR(loc) NEW_NODE(NODE_ERROR,0,0,0,loc)
 
 #define NODE_SPECIAL_REQUIRED_KEYWORD ((NODE *)-1)
 #define NODE_REQUIRED_KEYWORD_P(node) ((node)->nd_value == NODE_SPECIAL_REQUIRED_KEYWORD)