lakeFS: API with an unknown path should return an error

Currently lakeFS register openapi handlers and handle all specific routes. In case of a call to /api/v1/test, the unknown path under the API prefix, the mux will serve the request by the UI handler and return a valid HTML (UI) page.

The expected behaviour is to return a non-2xx status code with JSON error - prefered the internal error format, so the developer will handle an error and not fail to parse the response in case of a bad API call.