Seems we've drifted a bit from go1.12 support, mostly due to error
wrapping.
Fix this, as well as some assorted other failures.
I haven't tested 1.12 interactively.
For golang/go#39146
Change-Id: Id347ead2a13e89b76d2ae0047750e6b6b49911eb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/250941
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>