- Change response to be more central to Authboss. Make sure it has
useful methods and works with the new rendering idioms.
- Change the load user methods to all work with context keys, and even
be able to set context keys on the current request to avoid setting
contexts everywhere in the code base.
- Add error handling at the routing level so that all errors can bubble
up and be handled and logged there in one place.
- Add Err variants for ClientStorer and Attributes to facilitate
generating errors for missing type-failing arguments.
- Add better control flow and error handling for callbacks.
