libexpr: Fix invalid handling of errors for imported functions

c39cc00404 has added assertions for
all Value accesses and the following case has started failing with
an `unreachable`:

(/tmp/fun.nix):

```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.

Fixes #13448.

(cherry picked from commit 6e78cc90d3)

tests/functional/lang/eval-fail-missing-arg-import.nix

@@ -0,0 +1 @@
+import ./non-eval-trivial-lambda-formals.nix { }