lnd_test: print predErr instead of generic waitPredicate error

lnd_test.go

@@ -6522,7 +6522,7 @@ func testMultiHopReceiverChainClaim(net *lntest.NetworkHarness, t *harnessTest)
 		return true
 	}, time.Second*15)
 	if err != nil {
-		t.Fatalf("htlc mismatch: %v", err)
+		t.Fatalf("htlc mismatch: %v", predErr)
 	}
 
 	// Now we'll mine enough blocks to prompt carol to actually go to the