This is an array-like object that's useful for answer questions about the storage controllers arragement as a whole.
Create a #limit method in the StorageController class so we can customize the error message when a storage controller is full.
This makes it easier to check if the required controller can be found, and automatically raise an error if it is not. Add a #storage_bus method to the StorageController class as a shorthand way to check the general storage controller type.
