Require that InvalidParse nodes must have an error (#4482)

Unifies some boilerplate AddLeafNode calls for InvalidParse, and checks
has_error in AddNode.

toolchain/parse/handle_paren_condition.cpp

@@ -23,8 +23,7 @@ static auto HandleParenCondition(Context& context, NodeKind start_kind,
     // For an open curly, assume the condition was completely omitted.
     // Expression parsing would treat the { as a struct, but instead assume it's
     // a code block and just emit an invalid parse.
-    context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
-                        /*has_error=*/true);
+    context.AddInvalidParse(*context.position());
   } else {
     context.PushState(State::Expr);
   }