Add better warning

[Simone Poggiali]

Create a new app for a project, now run it in the browser but without having ActyxOS, nothing appears.
If you open console we get error:

webSocketSubject.js:69 WebSocket connection to 'ws://localhost:4243/store_api' failed: Error in connection establishment: net::ERR_CONNECTION_REFUSED

Maybe we can add a more friendly error message in condole dev tools, like "Please make sure ActyxOS is running" or similar

[Alexander Halemba]

This is done in the Pond already.