MRI: improve error printing

`Debug()` is CRuby-style and `showMsg` is RGSS-style. The `Debug()` output is safe, `showMsg` might be fragile.

I think the RGSS-style error report is safe as long as `#backtrace` isn't modified in a weird way. Then it could crash. But CRuby/MRI also crashes in some cases.

Should I make the code more defensive?