Add in check for billing error

[ltirrell]

Previously, a 402 status code was not specifically handled, and give a cryptic error message.

This may not be the best Error type or message. I used the message from this dict returned by _handle_response() when a 402 status occurred:

{
    "error": {
        "errorType": "BILLING_ERROR",
        "name": "PaymentRequired",
        "msg": "Please add a valid payment method at https://flipsidecrypto.xyz/api to continue using the Flipside SDK/API. If you have already added a credit card you may be seeing this message due to a failed payment (please check the status of your account at https://flipsidecrypto.xyz/api).",
    },
    "data": None,
    "status": "error",
}

Here is an example tracebackfor what the error looks like without catching a 402.
traceback.txt

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