htlcswitch: replace outgoing failure with interface

Add a FailureDetail interface which allows us have different kinds of
failures for link errors. This interface will be used to cover failures
that occur when on invoice payment, because the errors have already
been enumerated in the invoices package.

htlcswitch/switch.go

@@ -923,8 +923,8 @@ func (s *Switch) parseFailedPayment(deobfuscator ErrorDecrypter,
 			)
 
 			log.Errorf("%v: (hash=%v, pid=%d): %v",
-				linkError.OutgoingFailure, paymentHash, paymentID,
-				err)
+				linkError.FailureDetail.FailureString(),
+				paymentHash, paymentID, err)
 
 			return linkError
 		}
@@ -942,7 +942,8 @@ func (s *Switch) parseFailedPayment(deobfuscator ErrorDecrypter,
 			OutgoingFailureOnChainTimeout,
 		)
 
-		log.Info("%v: hash=%v, pid=%d", linkError.OutgoingFailure,
+		log.Info("%v: hash=%v, pid=%d",
+			linkError.FailureDetail.FailureString(),
 			paymentHash, paymentID)
 
 		return linkError