whisper: log errors on failed tests (#18134)

Debug traces to investigate a travis issue on MacOS

whisper/whisperv5/peer_test.go

@@ -139,7 +139,7 @@ func initialize(t *testing.T) {
 
 		err = node.server.Start()
 		if err != nil {
-			t.Fatalf("failed to start server %d.", i)
+			t.Fatalf("failed to start server %d. err: %v", i, err)
 		}
 
 		for j := 0; j < i; j++ {

whisper/whisperv6/peer_test.go

@@ -232,7 +232,7 @@ func initialize(t *testing.T) {
 func startServer(t *testing.T, s *p2p.Server) {
 	err := s.Start()
 	if err != nil {
-		t.Fatalf("failed to start the fisrt server.")
+		t.Fatalf("failed to start the first server. err: %v", err)
 	}
 
 	atomic.AddInt64(&result.started, 1)