Add status codes to responses

[Haegi]

Motivation

We noticed that this application always returns the status code 200. Even if the feature toggle couldn't be found or the service is not even there. This makes it hard to use this application as automated e2e test scenarios as we can not check for the status code but we have to check the body of the returned HTML content.

Solution

In this PR we added the status code 500 if the service instance is missing and 404 if the feature toggle couldn't be found. Everything else still returns 200.

Tagging my pair: @phil9909

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