souphttpsrc: error out properly when receiving data along with an error status

souphttpsrc: error out properly when receiving data along with an error status

When receiving an error code from the http server, such as 404,
data might be sent along with it, like a web page. We don't want
to output that data in this case, and we also want to pass the
FLOW_ERROR return back to the base class, so it can stop properly.

https://bugzilla.gnome.org/show_bug.cgi?id=678429