routing: interpret mpp timeout failues

This commit adds success mission control
results for all hops along the route in
a mpp timeout and takes no action for
the final hop along the route. This is a
temporary measure to prevent the default
logic from penalizing the final node while
we decide how to penalize mpp timeouts.

routing/result_interpretation.go

@@ -216,6 +216,22 @@ func (i *interpretedResult) processPaymentOutcomeFinal(
 		// deliberately. What to penalize?
 		i.finalFailureReason = &reasonIncorrectDetails
 
+	case *lnwire.FailMPPTimeout:
+		// TODO(carla): decide how to penalize mpp timeout. In the
+		// meantime, attribute success to the hops along the route and
+		// do not penalize the final node.
+
+		i.finalFailureReason = &reasonError
+
+		// If this is a direct payment, take no action.
+		if n == 1 {
+			return
+		}
+
+		// Assign all pairs a success result except the final hop, as
+		// the payment reached the destination correctly.
+		i.successPairRange(route, 0, n-2)
+
 	default:
 		// All other errors are considered terminal if coming from the
 		// final hop. They indicate that something is wrong at the