[dev.regabi] test: make run.go error messages slightly more informative

This is intended to make it easier to write/change a test
without referring to the source code to figure out what the
error messages actually mean, or how to correct them.

Change-Id: Ie79ff7cd9f2d1fa605257fe97eace68adc8a6716
Reviewed-on: https://go-review.googlesource.com/c/go/+/281452


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>