Wirecard: Capture error code in the response

Allowing customers to see the actual error code can help in two primary
areas:

* Communicating with the gateway's customer support about a given
transaction.
* Enabling customers to localize error messages if they
so desire, especially for an international gateway like Wirecard.

Now customers who care can access the error code in the
ActiveMerchant::Response params, which is a gateway specific hash of
response details.

Closes #1086.