The error coming out of matchGoImport does not differentiate between
having no imports, and having some invalid imports.

This some extra context to the error message to help debug these issues.

Fixes #16467

Change-Id: I3e9a119ed73da1bed5e07365be0221ea6b7f19db
Reviewed-on: https://go-review.googlesource.com/25121
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>