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.
Nope, I didn't even know about that convention. It'll probably be valuable to follow the google style guide where possible since we're using their discovery service. I'll get that up in a subsequent PR today.