Add user feedback for errors

[nesnoj]

Catch some important errors such as connection error "Failed to load resource: net::ERR_NETWORK_CHANGED" and provide error message

[henhuy]

When does error from above appear? You mean user feedback when simulating or in general?

[nesnoj]

I mean a general error feedback, e.g. in the case above the connection was lost (ajax error in console) but users may wonder why the app is not responding..