Add an error to `persistWithCompletion` about datastack HOT 5 CLOSED

I guess you prefer returning error instead of fatalError? I went for doing the fatalError thing because it the save fails it's really hard to recover from that error.

What do you have in mind?

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Well, I’m trying to write RAC extensions for the library, and it’s a bit hard to determine whether something succeeds or not and what path to take with no context. I’d prefer the error, because IMO, it’s the developer’s choice on whether or not to fail, especially on something like a save.

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Unless a failed save will corrupt the whole stack, I don't think it's the place of the framework to make that decision

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Makes sense 💐

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Added in #50

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