Provid more context when error is thrown

[Tudyx]

I'm trying to use the quizz inside my mdbook but sadly I'm panicking at this line:

https://github.com/willcrichton/mdbook-quiz/blob/main/src/processor.rs#L207

I think maybe we could have some context to the error, for instance the variable chapter to make it easier to debug.

[Tudyx]

I propose a solution for my problem here https://github.com/willcrichton/mdbook-quiz/pull/10

[willcrichton]

Closed by #10.