exp/template: ensure that a valid Set is returned even on error.
exp/template: ensure that a valid Set is returned even on error.

R=r
CC=golang-dev
http://codereview.appspot.com/4838044

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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