routing: continue trying after mpp timeout

It can happen that the receiver times out some htlcs of the set if it
took to long to complete. But because the sender's mission control is
now updated, it is worth to keep trying to send those shards again.

routing/result_interpretation_test.go

@@ -325,8 +325,10 @@ var resultTestCases = []resultTestCase{
 		failure:       &lnwire.FailMPPTimeout{},
 
 		expectedResult: &interpretedResult{
-			finalFailureReason: &reasonError,
-			nodeFailure:        nil,
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(0, 1): successPairResult(100),
+			},
+			nodeFailure: nil,
 		},
 	},
 
@@ -342,9 +344,9 @@ var resultTestCases = []resultTestCase{
 		expectedResult: &interpretedResult{
 			pairResults: map[DirectedNodePair]pairResult{
 				getTestPair(0, 1): successPairResult(100),
+				getTestPair(1, 2): successPairResult(99),
 			},
-			finalFailureReason: &reasonError,
-			nodeFailure:        nil,
+			nodeFailure: nil,
 		},
 	},
 }