Improve error message when AbstractSerializableMixin.from_data dispatches to itself

[drhagen]

AbstractSerializableMixin.from_data uses the _type attribute to look up the subclass and dispatch to its from_data. If the subclass does not have its own from_data method, then the dispatch comes right back to itself, but without the _type attribute and gives a weird error. An effort should be made to improve this error, maybe via introspection to see if the function being dispatched to is the same object as the current function.