inconsistent interface for sess.data_objects.get and sess.collecctions.get

When fetching a non-existant resource:

```
sess.collections.get ('/iplant/home/ZZZ')
```

raises CollectionDoesNotExist  

BUT a non-existant file gets a weird error

```
sess.data_objects.get ('/iplant/home/ZZZ')
```

AttributeError: 'iRODSDataObject' object has no attribute 'id'

Expected to get an exception DataObjectDoesNotExist
