Improve error reporting

[morgoth]

Currently when error is returned its body contains json as string https://github.com/paymill/paymill-ruby/blob/master/lib/paymill.rb#L50
It would much more helpful to have more structured error that contains fields from

{
    "exception":"subscription_not_found",
    "error":"Subscription not found"
}

[morgoth]

I found very weird case.
I'm using fake CC with 5105105105105100 to have error response as described in https://developers.paymill.com/guides/reference/testing.html

I get error paymill, during creating Payment::Transaction, but the body of response is a regular json of transaction, something like:

{
    "data":{
        "id":"tran_id",
        "amount":2990,
                ....
        }
}

Does anyone know what can be expected in response body of error?