onException: Shoud it allow modifying the path?

Currently we have:

```haskell
Di.Core.onException
  :: (SomeException -> Maybe (level, Seq path, msg))
  -> Di level path msg
  -> Di level path msg
```

But perhaps we want:

```haskell
Di.Core.onException
  :: (SomeException -> Maybe (level, msg))
  -> Di level path msg
  -> Di level path msg
```

Similarly in `Di.Monad.onException`.