further updates to the webserver start/stop functionality

implement waiting for the stop to complete
add pid tracking
update messaging to use timeout (interval) instead of deadline (a moment in time)
update naming of the constants
catch requests.ConnectionError in RayOnGolemClient._make_request and emit the unified RayOnGolemClientError instead
add error handling to RayOnGolemClient.non_terminated_nodes

golemfactory / ray-on-golem