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.
Interesting - the googleapi library seems to be slightly at odds with [this](https://developers.google.com/discovery/v1/using#build-call):
> The format of the errors is documented as part of the [Google JSON Style Guide](http://google-styleguide.googlecode.com/svn/trunk/jsoncstyleguide.xml#apiVersion)

which talks about a body having the more complex structure
```
{
  "apiVersion": "2.0",
  "error": {
    "code": 404,
    "message": "File Not Found",
    "errors": [{
      "domain": "Calendar",
      "reason": "ResourceNotFoundException",
      "message": "File Not Found
    }]
  }
}
```

Not really clear on the discrepancy