routing: exit on unexpected RequestRoute error

We whitelist a set of "expected" errors that can be returned from
RequestRoute, by converting them into a new type noRouteError. For any
other error returned by RequestRoute, we'll now exit immediately.

routing/mock_test.go

@@ -141,7 +141,7 @@ func (m *mockPaymentSession) RequestRoute(_, _ lnwire.MilliSatoshi,
 	_, height uint32) (*route.Route, error) {
 
 	if len(m.routes) == 0 {
-		return nil, fmt.Errorf("no routes")
+		return nil, errNoPathFound
 	}
 
 	r := m.routes[0]