NetconfTransportException should container reference to error reply from device

When a netconf device returns and error response <reply-error>, the protocol handler throws a NetconfTransportException.

• This really isn't an exception as an error response is not an unexpected response
• The exception doesn't contain the response XML so the caller can't really do anything intelligent. for example, if i an attempt to delete something that doesn't exists, the caller could parse the response see that it doesn't exists and move on