Notify a user when he loses connection with Workspace Agent

[vparfonov]

When a Workspace Agent (WS Agent) stop responding for whatever reason, projects disappear from project tree, and page refresh results in Cannot get project types error. On Workspace Master (WS MAster) workspace still marked as RUNNING.

For example, when OOM happens in a WS Agent, kernel may kill WS Agent process. Problem here that we can't detect properly OOM error but we can in some way check is WS Agent availability for user via HTTP request and WebSocket connection.

If client (IDE) lost WebSocket connection to the WS Agent. In this case IDE will request some other service in our infrastructure to check WS Agent state here we have two possibility for this situation:

1. WS Agent really shutdown by OS. If it not available for this service too, a user should be notified that the workspace is broken probably because of OOM (it will be just suggest because we not sure about reason). We can propose add more RAM and try to restart a workspace.
2. WS Agent working well and accessible for our infrastructure in this case we will inform user about networking problem. It can be not well configured proxy server our other problems not related to our responsibility. Here will also can give some suggest like ask your admin etc.

Here step to reproduce #1988