We should recur in `tryFlatten`

[glasserc]

If we throw an AnnotatedException [1] (AnnotatedException [2] (AnnotatedException [3] e)), we should get AnnotatedException [1, 2, 3] e. Instead we only flatten the toplevel exception so we get AnnotatedException [1, 2] (AnnotatedException [3] e).

[glasserc]

I think this is fine actually, because we call toException on the inner AnnotatedException, which also flattens it.