Got 'Undefined variable: sam' when using `g:vader_exception` right after `AssertThrows`

Hi folks.
I got this suspicious bug when doing a test. When I passed a variable `sam` defined within an `Execute` block to a function and tested the call with an `AssertThrows` and then wanted to verify the throw with `g:vader_exception`, Vader/Vim gave this error:
```vader
(2/2) [EXECUTE] Undefined sam
> Vim(call):E121: Undefined variable: sam
```
Here is a minimal example that repeats the situation:
```vader
Execute(Undefined sam):
function! F(x)
throw 'Hello'
endfunction

let sam = {}
AssertThrows call F(sam)
Log g:vader_exception
```
The variable `g:vader_throwpoint` also subjects to this bug.
```vader
Execute(Undefined sam):
function! F(x)
throw 'Hello'
endfunction

let sam = {}
AssertThrows call F(sam)
Log g:vader_throwpoint
```
shows:
```
(1/1) [EXECUTE] Undefined sam
> function vader#assert#throws, line 5
```

Have I been using the ``AssertThrows`` tool in the wrong way? Note, even if the throwing function is autoloaded, the bug is still here.