Also improve the error messages from a failing diagnostic tests so you can read
them.
Change-Id: I3554ce5a029de22a55a9636ed26ba02d95fc3246
Reviewed-on: https://go-review.googlesource.com/c/150042
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>