routing: also fail pairs for node-level failures

This commit modifies the interpretation of node-level failures.
Previously only the failing node was marked. With this commit, also the
incoming and outgoing connections involved in the route are marked as
failed.

The change prepares for the removal of node-level failures in mission
control probability estimation.

routing/result_interpretation_test.go

@@ -146,6 +146,10 @@ var resultTestCases = []resultTestCase{
 
 		expectedResult: &interpretedResult{
 			nodeFailure: &hops[1],
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(1, 0): failPairResult(0),
+				getTestPair(1, 2): failPairResult(0),
+			},
 		},
 	},
 
@@ -160,6 +164,9 @@ var resultTestCases = []resultTestCase{
 		expectedResult: &interpretedResult{
 			finalFailureReason: &reasonError,
 			nodeFailure:        &hops[1],
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(1, 0): failPairResult(0),
+			},
 		},
 	},
 }