lnwire+htlcswitch: only use references for failure messages

Methods on failure message types used to be defined on value receivers.
This allowed assignment of a failure message to ForwardingError both as
a value and as a pointer. This is error-prone, especially when using a
type switch.

In this commit the failure message methods are changed so that they
target pointer receivers.

Two instances where a value was assigned instead of a reference are
fixed.

lnwire/onion_error_test.go

@@ -177,7 +177,7 @@ func TestFailUnknownPaymentHashOptionalAmount(t *testing.T) {
 	// Creation an error that is a non-pointer will allow us to skip the
 	// type assertion for the Serializable interface. As a result, the
 	// amount body won't be written.
-	onionError := FailUnknownPaymentHash{}
+	onionError := &FailUnknownPaymentHash{}
 
 	var b bytes.Buffer
 	if err := EncodeFailure(&b, onionError, 0); err != nil {
@@ -189,7 +189,7 @@ func TestFailUnknownPaymentHashOptionalAmount(t *testing.T) {
 		t.Fatalf("unable to decode error: %v", err)
 	}
 
-	if !reflect.DeepEqual(onionError, onionError) {
+	if !reflect.DeepEqual(onionError, onionError2) {
 		t.Fatalf("expected %v, got %v", spew.Sdump(onionError),
 			spew.Sdump(onionError2))
 	}