Prevent weird error message
Prevent weird error message

Before, calling append on the wrong type of trajectory would give:

```
Error using append (line 38)
Wrong number of input arguments for obsolete matrix-based
syntax.
```

Because it would try matlab's built-in append.
billhoffman/drake,sheim/drake,psiorx/drake,psiorx/drake,sheim/drake,hanssusilo/drake,hanssusilo/drake,hanssusilo/drake,billhoffman/drake,sheim/drake,hanssusilo/drake,sheim/drake,billhoffman/drake,sheim/drake,crogers3/drake,crogers3/drake,crogers3/drake,psiorx/drake,billhoffman/drake,psiorx/drake,billhoffman/drake,psiorx/drake,hanssusilo/drake,hanssusilo/drake,psiorx/drake,sheim/drake,psiorx/drake,crogers3/drake,psiorx/drake,crogers3/drake,hanssusilo/drake,crogers3/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,hanssusilo/drake,sheim/drake,crogers3/drake,crogers3/drake