"OmniSharp server is not running" error with no hints on how to proceed

If you open an empty workspace and run command `.NET: Restore Project`, it will install various packages in the C# output window, then show an error message "OmniSharp server is not running" with no further info. It would be nice if this could direct users to open a project, check an output channel, or some other tip on how to proceed rather than just stating the error.