Handle Issue Voucher Failure

[StuartFerguson]

In ProcessSaleMessage of the Voucher Management Proxy when the Issue Voucher Command is dispatched the result is handled as follows:

`if (result.IsSuccess) {
// Build the response metadata
Dictionary<String, String> additionalTransactionResponseMetadata = new Dictionary<String, String>();
additionalTransactionResponseMetadata.Add("VoucherCode", result.Data.VoucherCode);
additionalTransactionResponseMetadata.Add("VoucherMessage", result.Data.Message);
additionalTransactionResponseMetadata.Add("VoucherExpiryDate", result.Data.ExpiryDate.ToString("yyyy-MM-dd"));

return Result.Success(new OperatorResponse
{
    TransactionId = transactionId.ToString("N"),
    ResponseCode = "0000",
    ResponseMessage = "SUCCESS",
    // This may contain the voucher details to be logged with the transaction, and for possible receipt email/print
    AdditionalTransactionResponseMetadata = additionalTransactionResponseMetadata,
    AuthorisationCode = "ABCD1234",
    IsSuccessful = true
});

}

// TODO: handle a failed issue case
return null;`

Some handling needs added to ensure that of the issue command fails that at minimum something useful is logged.

Need to also consider if the voucher has actually been issued (e.g. written to EventStore) as the transaction will be declined.