Point to the right call site when an exception is thrown

Review after https://github.com/allending/Kiwi/pull/317

I have in fact no idea how a lot of the internals in Kiwi work, so I'm going through them and figuring what's what. Occasionally I got the urge to fix small things. Here:

When an exception is thrown inside a spec, right now Xcode displays a generic failure not pointing anywhere, so you have to read the whole description and figure out where exactly the exception was raised. With this PR, the failure becomes the same as when a single line fails -- instead it shows up at the `it` block level. It doesn't do it that elegantly for other block types, but they should still be clickable.

![exception](http://i.imgur.com/nzAAzT2.png)