[P2P] merge_unpack does not reschedule on ShuffleClosedError

[hendrikmakait]

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.)