async proc always raises Exception according to raises pragma
So it looks like you still can't use `async` in combination with exception tracking. 

Any `proc` with `{.async, raises:[Defect].}` will result in:
`/home/deme/repos/nim-beacon-chain/vendor/nim-chronos/chronos/asyncmacro2.nim(63, 12) Error: can raise an unlisted exception: Exception`

E.g. even:
```nim
proc test() {.async, raises: [Defect].} =
    discard
```
This however doesn't:
```nim
proc test(): Future[void] {.raises: [Defect].} =
    discard
```

Next to that, it also seems that `future.complete()` can also raise `Exception`.