exclude error sources from meta serialization

Shouldn't change the behaviour of these on a patch release, but practically less than a handful of people would have used this so far and for most of those this particular option was likely irrelevant anyway.

I've been testing out the (de-)serialization feature more rigorously now in realistic use cases and changed my mind about including the error breakdown in the corresponding (de-)serialization methods: They were causing more problems than anything else for me and I found it easier to handle them separately altogether. (Realistically, one would mainly want to serialize the raw MC AOs from Rivet which typically wouldn't even have the error breakdown...)