htlcswitch: only error in closeCircuit if the htlc was failed

This commit changes the switch to only log an error if update_fail_htlc
comes in and closeCircuit returns ErrUnknownCircuit. Rationale
being that only settles should hit this code path, anything else
is a result of a link flap and should be treated as an error.

htlcswitch/switch_test.go

@@ -761,8 +761,8 @@ func TestSwitchForwardSettleAfterFullAdd(t *testing.T) {
 	}
 
 	// Send the settle packet again, which should fail.
-	if err := s2.forward(settle); err == nil {
-		t.Fatalf("expected failure when sending duplicate settle " +
+	if err := s2.forward(settle); err != nil {
+		t.Fatalf("expected success when sending duplicate settle " +
 			"with no pending circuit")
 	}
 }