Add raise to around callbacks

First I flipped the conditional to use if/else instead of unless/else since it's frowned upon to use an else with an unless. if/else is cleaner.

I added a raise if the `env.value` is `nil` because if `around_destroy` is called with `destroy` and the developer fails to add a `yield` to their callback an error will be thrown that records were not destroyed. This is confusing and a raise the explains that a yield is missing is more user friendly. 

I wasn't sure which error to throw, other callbacks were throwing an `ArgumentError` so I went with that. Let me know if that's incorrect and I'll fix it right up.