Should we allow to start the server even if the response file does not exist?

The `response-file` allows users to use a JSON file for declaring the response from the server. Currently we check if the file exists before starting the server and fail if it does not exist.

In one hand once we start the server we ready to receive traffic and we should be guarantee on start up that we are ready to do so. In the other hand, there is a difference between being up and being available and hence it is possible we could launch the container even before the file exists.

The technical implications of a potential change are really low.