New("x").ParseFiles("y") can result in an empty "x" template.
Make the message clearer that this is the problem. The error
returns from both template packages in this case were
confusing.

I considered making the method use "x" instead of "y" in
this case, but that just made other situations confusing
and harder to explain.

Fixes #2594.

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/5498048