fix(lib): check if an ERROR node is named before assuming it's the builtin error node

(cherry picked from commit b7f36a13ba)

test/fixtures/test_grammars/anonymous_error/corpus.txt

@@ -0,0 +1,9 @@
+======================
+A simple error literal
+======================
+
+ERROR
+
+---
+
+(document)

test/fixtures/test_grammars/anonymous_error/grammar.js

@@ -0,0 +1,6 @@
+module.exports = grammar({
+  name: 'anonymous_error',
+  rules: {
+    document: $ => repeat(choice('ok', 'ERROR')),
+  }
+});