- Centralized 'x is not mapped' reporting into sa.orm.exc.

- Centralized 'x is not mapped' reporting into sa.orm.exc.
- Guards are now present on all public Session methods and passing in an
  unmapped hoho anywhere yields helpful exception messages, going to some
  effort to provide hints for debugging situations that would otherwise seem
  hopeless, such as broken user instrumentation or half-pickles.