TUN-7073: Fix propagating of bad stream request from origin to downstream

This changes fixes a bug where cloudflared was not propagating errors
when proxying the body of an HTTP request.

In a situation where we already sent HTTP status code, the eyeball would
see the request as sucessfully when in fact it wasn't.

To solve this, we need to guarantee that we produce HTTP RST_STREAM
frames.
This change was applied to both http2 and quic transports.

proxy/proxy.go

@@ -261,7 +261,9 @@ func (p *Proxy) proxyHTTPRequest(
 		return nil
 	}
 
-	_, _ = cfio.Copy(w, resp.Body)
+	if _, err = cfio.Copy(w, resp.Body); err != nil {
+		return err
+	}
 
 	// copy trailers
 	copyTrailers(w, resp)