There is an example here how to process the body of a HTTP request: http://twistedmatrix.com/documents/12.2.0/web/howto/client.html#auto4
The description here say that once a GET request is made then have to read the body or the connection will remain open:
An important thing to keep in mind is that the body will only be read from the connection after Response.deliverBody is called. This also means that the connection will remain open until this is done (and the body read). So, in general, any response with a body must have that body read using deliverBody. If the application is not interested in the body, it should issue a HEAD request or use a protocol which immediately calls stopProducing on its transport.
However if the headers indicate an error then there is no need to read the body. In this case how can the body be ignored without leaving the connection open?