sniblocking: interrupted is a more proper error here

When the experiment was written, we didn't have this error. Now that we
have it, we should use it, because it's more proper.

Part of #684

experiment/sniblocking/sniblocking.go

@@ -114,7 +114,7 @@ func (m *measurer) measureone(
 	select {
 	case <-time.After(sleeptime):
 	case <-ctx.Done():
-		s := modelx.FailureGenericTimeoutError
+		s := modelx.FailureInterrupted
 		return Subresult{
 			TestKeys: urlgetter.TestKeys{
 				Failure: &s,

experiment/sniblocking/sniblocking_test.go

@@ -201,7 +201,7 @@ func TestUnitMeasureoneCancelledContext(t *testing.T) {
 	if result.FailedOperation != nil {
 		t.Fatal("not the expected FailedOperation")
 	}
-	if result.Failure == nil || *result.Failure != modelx.FailureGenericTimeoutError {
+	if result.Failure == nil || *result.Failure != modelx.FailureInterrupted {
 		t.Fatal("not the expected failure")
 	}
 	if result.NetworkEvents != nil {