- 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.

We use the same nested map representation we used for goals, again in
order to save space. We might someday want to combine with `inputDrvs`,
by doing `V = bool` instead of `V = std::set<OutputName>`, but we are
not doing that yet for sake of a smaller diff.

The ATerm format for Derivations also needs to be extended, in addition
to the in-memory format. To accomodate this, we added a new basic
versioning scheme, so old versions of Nix will get nice errors. (And
going forward, if the ATerm format changes again the errors will be even
better.)

`parsedStrings`, an internal function used as part of parsing
derivations in A-Term format, used to consume the final `]` but expect
the initial `[` to already be consumed. This made for what looked like
unbalanced brackets at callsites, which was confusing. Now it consumes
both which is hopefully less confusing.

As part of testing, we also created a unit test for the A-Term format for
regular non-experimental derivations too.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

Apply suggestions from code review

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

The Derivation parser and old ATerm unfortunately leaves few ways to get
nice errors when an old version of Nix encounters a new version of the
format. The most likely scenario for this to occur is with a new client
making a derivation that the old daemon it is communicating with cannot
understand.

The extensions we just created for dynamic derivation deps will add a
version field, solving the problem going forward, but there is still the
issue of what to do about old versions of Nix up to now.

The solution here is to carefully catch the bad error from the daemon
that is likely to indicate this problem, and add some extra context to
it.

There is another "Ugly backwards compatibility hack" in
`remote-store.cc` that also works by transforming an error.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>