Creating clearer KeyError messages.

[MicahGale]

This is a quick first pass at trying to make the KeyError message clear this is a user error, and not a library error. @paulromano, Is this a reasonable approach before I go further down this path?

Things implemented:

1. Moved all manually documented attributes to @property wrappers for _private fields.
2. Made a subclass of dict. This overwrites __getitem__ to rewrite the KeyError.
3. Tried to disable __setitem__ for the wrapper to make it immutable. Not sure if I did this right.
4. Wrapper is spawned as a deepcopy of the _private dicts to add a layer of immutability as well.

TODO

• Verify this behaves as expected
• Check on test coverage
• Clarify type annotations are accurate.

Fixes #17