MutateIn is not throwing and classifying sub-doc errors correctly.

I think this has gone both directions. We need to nail down what the correct behavior is according to the RFC.

Currently, a MutateIn with StoreSemantics.Upsert does not throw PathAlreadyExists on an XATTR with CreatePath: false, even when the server is returning SubDocPathExists.  It just silently succeeds, and there's no way for the user to detect it via any public fields/properties/methods on the return value.

Transactions RFC seems to imply that it should be throwing.