Improve error response unmarshalling

[EVODelavega]

Looking through some of the code, I noticed errors were manually unmarshalled using a ton of type assertions and maps. That's not good. The problem was that the error types don't export their underlying stripeErr fields. json.Unmarshal uses reflection and tags to find fields that are exported (otherwise it can't set values). I've created an un-exported rawErr type that can be used for unmarshalling, and a further type internalError that embeds the Error type as it is, and adds the decline_code field at the same level (to correspond to the error format).

Ideally, the decline_code field should be a part of the main Error type, so we don't need that second internal type. The type of decline_code was (and is) string. I've left it as is to not break any existing code, but if a field is tagged as omitempty, a pointer type should be used.

Tests all pass, no behavioural changes. The code is just a bit less verbose, more readable, easier to maintain, less error-prone, and possibly faster (though I didn't write a Bench test).

[brandur-stripe]

Thanks @EVODelavega! Looks like an improvement to me.

I added some review comments below, but the general approach looks fine.

[brandur-stripe]

LGTM. Thanks!

[brandur-stripe]

Released as 53.2.0.