Gate the onMetrics plugin hook behind a boolean that's set only after
the Factorio server has fully started, and reset when it stops. This
prevents errors due to plugins trying to get metrics while the server
is creating a new save or stopped.
Ensure that notifyExit is called if errors occur during startInstance
and createSave request, and rework startInstance save logic to create
a new savegame if no save is present and allow specifying the save to
load.
Pull the exit handler into its own method that's invoked if the startup
fails before the Factorio server is created. Also move the metrics
clearing to the exit notification method to ensure it's also invoked.
Hornwitser