Manifest server returns no meaningful error

[georgkrause]

If for some reason the manifest service is unable to generate the manifests, it simply returns a 502 server error without any meaningful information. This means that users of Unikube are not able to fix the problem in their charts. There might be some distinguation required if the error should be returned (errors in the charts) or not (eg errors in the unikube services) to avoid leaking sensible information.