Exit code 3 is unprecedented and inconsistent with other failures here,
such as having no tool directory.

Fixes #17145

Change-Id: Ie7ed56494d4511a600214666ce3a726d63a8fd8e
Reviewed-on: https://go-review.googlesource.com/31253Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>