ResponseDone should be a subclass of ConnectionDone

[davidsarah]

twisted.web.[_new]client.ResponseDone should be a subclass of twisted.internet.error.ConnectionLost, so that when it is passed to the connectionLost method of a protocol, the API for IProtocol.connectionLost is followed.

This should not have any backward compatibility impact, I think (ResponseDone currently directly subclasses Exception).

[Itamar Turner-Trauring]

I think the interface documentation should change, rather than the code. First, we have other places where exceptions that don't inherit from ConnectionLost are passed to the protocol. Second, it's not clear to me what benefit inheriting from ConnectionLost gives you.

Can you give an example use case motivating this change?

[Jean-Paul Calderone]

Future readers might observe that ConnectionLost is not the same as ConnectionDone. I think itamar meant to write ConnectionDone, since that's what the ticket description actually proposes.