net: use original argument in lookup error
net: use original argument in lookup error

Fixes issue 6324.

R=golang-dev, iant, mikioh.mikioh
CC=golang-dev
https://codereview.appspot.com/13280049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang