Add pickle support for UnwrapFailedError

## Problem Currently, the `UnwrapFailedError` class cannot be pickled (serialized) by default because it uses `__slots__`. This creates issues when dealing with this exception in multiprocessing or other distributed environments. ## Proposal Add a `__reduce__` method to the `UnwrapFailedError` class to make it picklable. This will allow the exception to be serialized/deserialized while preserving the `halted_container` information.