libexpr: Fix invalid handling of errors for imported functions (backport #13450)

[mergify]

Motivation

c39cc00 has added assertions for all Value accesses and the following case has started failing with an unreachable:

(/tmp/fun.nix):

{a}: a

$ nix eval --impure --expr 'import /tmp/fun.nix {a="a";b="b";}'

This would crash:

terminating due to unexpected unrecoverable internal error: Unexpected condition in getStorage at ../include/nix/expr/value.hh:844

This is not a regression, but rather surfaces an existing problem, which previously was left undiagnosed. In the case of an import fun is the import primOp, so that read is invalid and previously this resulted in an access into an inactive union member, which is UB. The correct thing to use is vCur. Identical problem also affected the case of a missing argument.

Add previously failing test cases to the functional/lang test suite.

Context

Fixes #13448.

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This is an automatic backport of pull request #13450 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 6e78cc9 has failed:

On branch mergify/bp/2.28-maintenance/pr-13450
Your branch is up to date with 'origin/2.28-maintenance'.

You are currently cherry-picking commit 6e78cc90d.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	new file:   tests/functional/lang/eval-fail-missing-arg-import.err.exp
	new file:   tests/functional/lang/eval-fail-missing-arg-import.nix
	new file:   tests/functional/lang/eval-fail-undeclared-arg-import.err.exp
	new file:   tests/functional/lang/eval-fail-undeclared-arg-import.nix
	new file:   tests/functional/lang/non-eval-trivial-lambda-formals.nix

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libexpr/eval.cc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

This pull request has been removed from the queue for the following reason: pull request dequeued.

Pull request #13451 has been dequeued. The pull request could not be merged. This could be related to an activated branch protection or ruleset rule that prevents us from merging. (details: Required status check "installer test on macos" is in progress.).

You should look at the reason for the failure and decide if the pull request needs to be fixed or if you want to requeue it.
If you do update this pull request, it will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.