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.

htlcswitch/switch.go

@@ -981,7 +981,7 @@ func (s *Switch) parseFailedPayment(deobfuscator ErrorDecrypter,
 		return &ForwardingError{
 			FailureSourceIdx: 0,
 			ExtraMsg:         userErr,
-			FailureMessage:   lnwire.FailPermanentChannelFailure{},
+			FailureMessage:   &lnwire.FailPermanentChannelFailure{},
 		}
 
 	// A regular multi-hop payment error that we'll need to