Option to suppress exceptions with templatetags 

I have code like this in the template:

```
{% thumbnail '588x344' idea.image as thumb %}
<img src="{{ thumb.url }}" alt="{{ idea.title }}" image-ratio />
```

This will cause a 500 error if the source image is missing. This commonly happens during development when the database and media dir get out of sync.

IMHO there should be a way for this to easily return a 404, converting it from an error which completely breaks the page into a more appropriate 'broken image' icon from the browser.