clarify the comment

and use the same error message for CREATE and open table time

sql/table.cc

@@ -2589,10 +2589,10 @@ static bool fix_and_check_vcol_expr(THD *thd, TABLE *table, Field *field,
   /* fix_fields could've changed the expression */
   func_expr= vcol->expr_item;
 
-  /* Number of columns will be checked later */
+  /* this was checked in check_expression(), but the frm could be mangled... */
   if (unlikely(func_expr->result_type() == ROW_RESULT))
   {
-     my_error(ER_ROW_EXPR_FOR_VCOL, MYF(0));
+    my_error(ER_OPERAND_COLUMNS, MYF(0), 1);
     DBUG_RETURN(1);
   }