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.
I was just coding to the go client bindings, but I did just realize I made a mistake. The entities we return need to be in an {"error": {...}} container. That'll match up to the style guide like you point out. Will fix before I land this PR.