feat(API): Return error entities on bad page tokens

This is the first major wart of the google discovery schema stuff. It defines this convention for serializing error entities, yet it does not serve the schema as part of the schema itself. It seems like we just have to live with this.
Correspondingly do the the 2xx responses do have a `data` container? If so, maybe  we should update the "cats" example above to be more clear.