The default error printer shows `#message`, `#additional_message`, and a
backtrace.

```ruby
class MyError < StandardError
  def message = "my error!"
  def additional_message = "This is\nan additional\nmessage"
end

raise MyError
```

```
$ ./miniruby test.rb
test.rb:6:in `<main>': my error! (MyError)
| This is
| an additional
| message
```

Also, let did_you_mean and error_highlight override the method instead
of `#messsage` experimentally.