net/dns/resolver: update (*forwarder).forwardWithDestChan to always return an error unless it sends a response to responseChan

We currently have two executions paths where (*forwarder).forwardWithDestChan
returns nil, rather than an error, without sending a DNS response to responseChan.

These paths are accompanied by a comment that reads:
// Returning an error will cause an internal retry, there is
// nothing we can do if parsing failed. Just drop the packet.
But it is not (or no longer longer) accurate: returning an error from forwardWithDestChan
does not currently cause a retry.

Moreover, although these paths are currently unreachable due to implementation details,
if (*forwarder).forwardWithDestChan were to return nil without sending a response to
responseChan, it would cause a deadlock at one call site and a panic at another.

Therefore, we update (*forwarder).forwardWithDestChan to return errors in those two paths
and remove comments that were no longer accurate and misleading.

Updates #cleanup
Updates #13571

Signed-off-by: Nick Hill <mykola.khyl@gmail.com>

net/dns/resolver/tsdns_test.go

@@ -1503,8 +1503,8 @@ func TestServfail(t *testing.T) {
 	r.SetConfig(cfg)
 
 	pkt, err := syncRespond(r, dnspacket("test.site.", dns.TypeA, noEdns))
-	if !errors.Is(err, errServerFailure) {
-		t.Errorf("err = %v, want %v", err, errServerFailure)
+	if err != nil {
+		t.Fatalf("err = %v, want nil", err)
 	}
 
 	wantPkt := []byte{