Adds OAuth failure callback

This PR adds a simple URL for a callback on authentication failure. It adds a simple error message and redirects to homepage.

If you want to try it locally, you need to add the code below before the configuration of OmniAuth in app.rb, otherwise OmniAuth will raise an exception in development mode and won't redirect to the callback

```
OmniAuth.config.on_failure = Proc.new { |env|
  OmniAuth::FailureEndpoint.new(env).redirect_to_failure
}
```