Implement Error Tracking API

Hi! Would be helpful if the project supported [the `error_tracking` API](https://docs.gitlab.com/ee/api/error_tracking.html), especially now that [integrated error tracking is in open beta](https://docs.gitlab.com/ee/operations/error_tracking.html#integrated-error-tracking). In particular I'd like to extract the `sentry_dsn`, so we can start using the DSN without manually copying values.