Also, remove output file if there are encoding errors.
The extra reports are convenient.
Removing the output file is very important.
Noticed while testing.

Change-Id: I0fab17d4078f93c5a0d6d1217d8d9a63ac789696
Reviewed-on: https://go-review.googlesource.com/18845Reviewed-by: Rob Pike <r@golang.org>