Improve derivation parsing

- Don't assert: Derivation ATerms are not necessarily produced by Nix,
  and parsers should always throw graceful errors

- Improve error message from `static void except(..)`, shows both what
  we expected and what we actually got.

The intention is that we backport it, and then hopefully a few people
might get slightly better errors if they try out new experimental drv
files (for RFC 92) with an old version of Nix.

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