[P2P] `merge_unpack` does not reschedule on `ShuffleClosedError`

I just noticed that `merge_unpack` (https://github.com/hendrikmakait/distributed/blob/9fae5dacf4d2cfd5c659e472b0a3ef307d695863/distributed/shuffle/_merge.py#L163-L196) isn't wrapped in `handle_unpack_errors` like `shuffle_unpack` (https://github.com/hendrikmakait/distributed/blob/9fae5dacf4d2cfd5c659e472b0a3ef307d695863/distributed/shuffle/_shuffle.py#L94-L101).

This could cause P2P merging not to restart in some edge cases. (I'm not sure which ones, but there should be tests for P2P shuffling that will start to fail if we remove `handle_unpack_errors`.)