Stray error message

When calling 

```
puppet catalog diff --content_diff --show_resource_diff --render-as json old new
```

where `old` and `new` contain the pson-catalogs from old and new config, a stray error message 

```
Error: Add --debug for realtime output, add --render-as {json,yaml} for parsed output
```

gets display.

The diff gets created fine.
