Add access to error.data

[tuomassalo]

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