Maybe it will say something that helps the user understand the problem.

Note that we can't use os/exec.ExitError's new Stderr field because
cmd/dist is compiled with Go 1.4.

Fixes #13099.

Change-Id: I4b5910434bf324d1b85107002a64684d8ba14dc8
Reviewed-on: https://go-review.googlesource.com/17940Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>