Nice stuff.

One nitpick: it always feel weird to me to have an error (or any other object) where the attributes are named like this:

    {
        "errorCode": "E0000001",
        "errorSummary": "Api validation failed",
        "errorLink": "E0000001",
        "errorId": "oaeHfmOAx1iRLa0H10DeMz5fQ",
        "errorCauses": [
            {
                "errorSummary": "login: An object with this field already exists in the current organization"
            }
        ]
    }

Why have "error" in there at all?

To make it absolutely obvious everywhere in your code that you're talking about an errorSummary, not just a summary?