generatePID + failOnError fixes

Hey,

Thanks for creating this! Helped me setup an auto spawn system for a node server I had.

Though, I hadn't setup a detached system on my node system so I thought it would be cool for grunt-service to be able to handle that case and auto-generate a pid file for it and take it to the background.

I've also made sure that failOnError actually works if it's set – it seems like it wasn't fully done before.