Clarified exception msg when render fails

Catches exceptions when rendering uris, and improves their message a bit:

```
The URI ez_comments.controller.comments_renderer:renderForContent couldn't be rendered
```
instead of 
```
The controller for URI "/_fragment" is not callable.
```

P.S. This was a fast fix, see https://plus.google.com/u/0/115490809384399992762/posts/eS5YknUgXaU ;-)