Unauthorized GitHub should raise

[ys]

This PR introduce a raise_error middleware for Faraday GitHub client.
It will raise on 401 a new Escobar::Client::Error::Unauthorized.
Maybe we want to specialize errors per provider: GitHub or Heroku to differentiate them higher in the stack.

Why?

We raise RepoNotFound when the token has been revoked and return not accurate responses to users.

Another solution?

Use Octokit instead of this custom code.
That would allow us to reuse Octokit Client at places that do not fit the Escobar way:)

[ys]

[Slash-heroku] Escobar hides 401 when revoking tokens