Reading the couchdbkit code, it does seem to throw an error if you try to save a doc with the wrong `doc_type` via `DocumentBase.save_docs` but I could not quickly ascertain what occurs when you `DocumentBase.get(id)` and the retrieved document has the wrong `doc_type`. If the behavior is to simply instantiate mismatched docs, then we may as well throw a new sort of exception. Otherwise, my reasoning is that _in the eyes of this module_ the requested doc does not exist, and we should throw whatever exception we use to represent that situation.