%r format prints nothing useful on windows (see issue 9722).
Hopefully this will provide more clues about what happened.

Change-Id: Ic553bbdcde0c3cbfffa3a28f2168d6e75694e2ac
Reviewed-on: https://go-review.googlesource.com/3568Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>