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)

src/libexpr/eval.cc

@@ -1564,7 +1564,7 @@ void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes,
                                              symbols[i.name])
                                     .atPos(lambda.pos)
                                     .withTrace(pos, "from call site")
-                                    .withFrame(*fun.payload.lambda.env, lambda)
+                                    .withFrame(*vCur.payload.lambda.env, lambda)
                                     .debugThrow();
                         }
                         env2.values[displ++] = i.def->maybeThunk(*this, env2);
@@ -1591,7 +1591,7 @@ void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes,
                                 .atPos(lambda.pos)
                                 .withTrace(pos, "from call site")
                                 .withSuggestions(suggestions)
-                                .withFrame(*fun.payload.lambda.env, lambda)
+                                .withFrame(*vCur.payload.lambda.env, lambda)
                                 .debugThrow();
                         }
                     unreachable();