Gracefully recover from failure to detect the language server project in sibling directory

[rgrunber]

Our CONTRIBUTING.md server-side setup talks about how to set up vscode-java with eclipse.jdt.ls (placing them as sibling directories) in order to ensure npm run build-server builds eclipse.jdt.ls and uses the resulting content in the server/ folder of vscode-java.

However, we also have a gulp task that fetches the latest pre-built language server (eclipse.jdt.ls) from http://download.eclipse.org/jdtls/snapshots/jdt-language-server-latest.tar.gz . It would be good to integrate this workflow in the case a user has not properly set up a sibling directory.

I don't know if I'd do this for npm run build-server though maybe we create a separate task called npm run build that either performs build-server if eclipse.jdt.ls is detected, or the download_server task.