Router: Send Nginx errors to Sentry

[nokome]

We use Sentry for exception monitoring and alerting in the Hub's various services e.g. manager, worker, router. However, in the router the approach is a little hackish, requiring the serving of a custom error page and Javascript. An alternative approach is to have another process monitoring the Nginx logs e.g. https://pypi.org/project/SentryLogs/.

The downside of that is having another process running in the container. The upside being that we can be more certain that errors there get reported.