Add a more helpful error message to `ListObject#[]`

Our list calls return their results wrapped in an object so that we
can include extra information. We use this, e.g., to include the URL
to query for more records in the Transfer#transactions sublist.

When you get a ListObject, if you want to actually manipulate it as a
list, you have to call `#data` first to get the actual underlying
list.

This adds an exception to the `#[]` method to make what's going on
clearer.

Fixes #68