Inform the UI when we can't satisfy the HTTP Request and change the name of RouteQueryMessage to RouteQueryRequest.

[czarte]

In actual state, we do not inform UI when we can't find the Route to host. In ServerImpersonator we have 3 errors we can receive from the neighborhood, and we transform them into a simple web page that we send back to the browser.

The node/src/proxy_server/mod.rs handle_route_failure is the right place, where we want to inform UI about the RouteQueryRequest failure. That means we want to add a UIGateway subscriber into struct ProxyServer.

RouteQueryMessage now returns Option. Changing the return type would be beneficial in the way, that we will be able to write stronger tests for the RouteQuery Requests in case the Neighborhood fails to find a route.
RouteQueryMessage would probably be better to rename to RouteQueryRequest, which is more comprehensive in the matter of what the functionality is doing. It corresponds with our other Actix Messages, where we use Request and Response for particular data.