When the API is called with a revoked or invalid token, the error
handler did not properly return a `Rack::Response`, causing Grape to
throw an error in the middleware. Instead of a 4xx error code, the
client would receive a 500 error.

We fix this by recreating the Rack::Response after the
`Rack::OAuth2::Server::Abstract::Error#finish` call. The `yield` is
intercepted by the superclass, so the API handler never got it.

Relates to https://gitlab.com/gitlab-com/gl-infra/production/-/issues/2363