Handle errors returned by Trustly

According to the JSON RPC 1.1 specification, the Procedure Return object must contain the error number when a remote procedure call fails.

Example of a JSON object returned by Trustly when an error occurs:

    "version": "1.1",
    "error": {
        "name": "JSONRPCError",
        "code": 616,
        "message": "ERROR_INVALID_CREDENTIALS",
        "error": {
            "signature": "R9+hjuMqbsH0Ku ... S16VbzRsw==",
            "uuid": "258a2184-2842-b485-25ca-293525152425",
            "method": "Deposit",
            "data": {
                "code" : 616,
                "message" : "ERROR_INVALID_CREDENTIALS"

The message returned is not in a suitable format to present for customers.
If the error is JSON-RPC specific (for example invalid JSON-format), uuid and method will be "null".