This provides better diagnostics when it occurs.

Updates #19751

Change-Id: I87db54c22e1345891b418c1741dc76ac5fb8ed00
Reviewed-on: https://go-review.googlesource.com/39358
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>