Ch8 Finale task definition

[skaunov]

Can we improve task definition so that limitations would be obvious from the start? It's my bad, I wasn't reading thorough enough, but staring long enough at Matcher::new signature output I decided that there's no way it could return None. Implemented it the way unmatched parenthesis go to RawText(_), and of course smashed into broken_matcher() test.

It's just a piece of feedback... Do you feel like that this detail deserve few words in the chapter?