Missing error handling for API statuses and missing response code in result

[szilardd]

1. The SDK doesn't seem implement all status codes that are specified in the API
   https://reference.assemblypayments.com/#error-codes

For example if an error 500 or 408 occurs, what will happen in the SDK, will it throw an exception which has to be handled differently than the other type of errors that throw an ApiErrorsException ?

Would be good to handle all in the same way

2. Only statuses 400, 422 are handled and these are not provided in the ApiErrorsException class, you have to rely on the error message to determine the type of error if you want to give meaningful error messages to your users.

Would be good to include the status code at least in the ApiErrorsException object.

https://github.com/AssemblyPayments/promisepay-dotnet/blob/master/PromisePayDotNet/Implementations/AbstractRepository.cs#L113

3. Status 400 (BadRequest) is not documented in the API error codes page, though it is handled in the SDK.

https://github.com/AssemblyPayments/promisepay-dotnet/blob/master/PromisePayDotNet/Implementations/AbstractRepository.cs#L115

https://reference.assemblypayments.com/#error-codes

Would be good to document it in the API error codes page as well to be consistent