Don't throw exception from ContentAtPath

`Container`'s `ContentAtPath` currently throws an exception if it fails, which creates some rather ugly `try/catch` code in a number of places where we want to be able to fail gracefully if the content can't be found.

Convert `ContentAtPath` to return null if it doesn't find the content, and throw exceptions at the call site where it's still intended. (Such as in the main `Step()` function).

Note that this should be relatively straightforward if anyone wants to give it a try! (Though not before #210 is fixed so you can verify your code!)