Better handling when no compiler can be used to start lsp in vscode

[RodgeFu]

Now we are fallback to use tsp-server(.cmd) anyway if local compiler can't be found. This will cause our extension to report multiple confusing errors from vscode lsp client directly if global compiler is not available either. We should be able to handle that scenario better by checking whether tsp-server(.cmd) is available and prompt user to install local compiler to provide a smoother user experience in that case.