routing: remove errNoRoute and lastError

Now that SendToRoute is no longer using the payment lifecycle, we
remove the error structs and vars used to cache the last encountered
error. For SendToRoute this will now be returned directly after a shard
has failed.

For SendPayment this means that the last error encountered durinng
pathfinding no longer will be returned. All errors encounterd can
instead be inspected from the HTLC list.

routing/router_test.go

@@ -792,10 +792,33 @@ func TestSendPaymentErrorPathPruning(t *testing.T) {
 
 	// The final error returned should also indicate that the peer wasn't
 	// online (the last error we returned).
-	if !strings.Contains(err.Error(), "UnknownNextPeer") {
+	// TODO: proper err code
+	if !strings.Contains(err.Error(), "unable to find") {
 		t.Fatalf("expected UnknownNextPeer instead got: %v", err)
 	}
 
+	// Inspect the two attempts that were made before the payment failed.
+	p, err := ctx.router.cfg.Control.FetchPayment(payHash)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(p.HTLCs) != 2 {
+		t.Fatalf("expected two attempts got %v", len(p.HTLCs))
+	}
+
+	// We expect the first attempt to have failed with a
+	// TemporaryChannelFailure, the second with UnknownNextPeer.
+	msg := p.HTLCs[0].Failure.Message
+	if _, ok := msg.(*lnwire.FailTemporaryChannelFailure); !ok {
+		t.Fatalf("unexpected fail message: %T", msg)
+	}
+
+	msg = p.HTLCs[1].Failure.Message
+	if _, ok := msg.(*lnwire.FailUnknownNextPeer); !ok {
+		t.Fatalf("unexpected fail message: %T", msg)
+	}
+
 	ctx.router.cfg.MissionControl.(*MissionControl).ResetHistory()
 
 	// Next, we'll modify the SendToSwitch method to indicate that the