channeldb: return correct error when unable to find node
channeldb: return correct error when unable to find node

bryanvu/lnd,bryanvu/lnd,LightningNetwork/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd