parseResponse() guards against minimal range of error scenarios

[dcentrica]

In Client.php:parseResponse() - the only failure that's catered for is an HTTP 401, however in my case, using a legacy BitPay key pair and patching to use a BTCPayServer keypair, and not updating the server itself, I get an HTTP 403 - the logic should be patched to cater to success codes or all possible (likely) failure codes e.g. 401 and 403.