- follow wording of cmd/compile more closely
- only print base of a package path to avoid overly long error messages

Fixes #26234.

Change-Id: I47a8c64b3adcf73980cd296a24cf8ac721e5df06
Reviewed-on: https://go-review.googlesource.com/c/152764
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>