Avoid rescuing StandardError and Exception

This includes postfix rescues like:

```ruby
tree = Model.to_tree rescue []
```
Although they have clean syntax, they hide real errors and make debugging more difficult.

It's also noticeable that rescuing `Exception` [broadens the match](http://stackoverflow.com/questions/10048173/why-is-it-bad-style-to-rescue-exception-e-in-ruby#answer-10048406).