Backwards compatibiliy for 1.x release

[eval]

Before this commit when handling OmniKassa2::OmnikassaError it might have handled Omnikassa2::HttpError's.
This might have been the (unlikely) case where there's an unsuccessful response containing valid JSON.
With HttpError no longer inheriting from OmnikassaError this would break. Now ApiError takes it place until 2.0.

A deprecation warning is shown (also to those handling JSON::ParserError) to migrate to HttpError or OmnikassaError before 2.0.

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[rbr]

Wouldn't it be easier to just release the changes on master as a 2.0 release?
What's the added benefit of making a 1.1 for which there might be some risk of breaking something somewhere?

(I appreciate the effort put in to cover the situation we see, but with all the changes I'd think it's easier to not bother with a 1.x release.)