Add access to error.data

When a JSON-RPC call fails, the server *may* send additional information in  `error.data`. This change allows access to that data. Now you can do something like:

    s = jsonrpclib.Server('http://...')
    try:
      s.my.method.call(...)
    except AppError as e:
      # access e.data() here
    except:
      # protocol errors and other stuff go here, as earlier