Create speaking error message when a route with a too-new api version is requested

[fm3]

Assume v5 is the current api version. The request handler could check incoming api requests if they request v6,v7,v8 and if so, reply with something like "This WEBKNOSSOS instance does not yet support this API version"
A comment should be added in webknossos.versioned.routes to remind us of updating the code in the request handler when a new version is introduced