improved getattr template filter
I tried to use [Joshua's](http://www.djangosnippets.org/users/joshua/) nice and very useful [getattr template filter (#38)](http://www.djangosnippets.org/snippets/38/), but ran into a few problems. I used it on objects outside of my control (admin internals, coughcough) and on some of them the code didn't catch the resulting exceptions. So I improved the error handling a bit. Furthermore, the code now also returns the *value of a callable* instead of the callable *itself* (last 4 lines). Looking at my code though, it can certainly be improved further.
- template
- filter
- get
- attr