Prevent silent failures and recover after error.

Refs #4543
- Alway return model out of save handler so sub-classes which
  call this._super() on save have a model on success and failure.
- Always return resolved promise out of post slug generator.
- Test the errors object in the error notification helper to avoid
  throwing an unhandled exception and silently failing.
- Consider a post model "dirty" if it is in an error state.