refactor(API): Use custom errorResponse struct

The googleapi pkg provides an Error entity, but it
unnecessaryily serializes a field we do not use.
Creating an equivalent struct that we maintain
works around the problem while retaining the ability
for the client bindings to use googleapi.Error.
They aren't in the discovery document only because I don't know how to make that work. The entity is outlined in our documentation, which I believe "counts".