Improve errors / codes for node-session api.
Improve errors / codes for node-session api.

brocaar/lora-app-server,jcampanell-cablelabs/lora-app-server,jcampanell-cablelabs/lora-app-server,brocaar/lora-app-server,jcampanell-cablelabs/lora-app-server,brocaar/lora-app-server,jcampanell-cablelabs/lora-app-server,brocaar/lora-app-server