cmd/go: adjust error for custom import checkout mismatch

Before:

	...
	imports golang.org/x/net/context: /Users/rsc/g/src/golang.org/x/net is from https://code.google.com/p/go.net, should be from https://go.googlesource.com/net

After:

	...
	imports golang.org/x/net/context: golang.org/x/net is a custom import path for https://go.googlesource.com/net, but /Users/rsc/g/src/golang.org/x/net is checked out from https://code.google.com/p/go.net

Change-Id: I93c35b85f955c7de684f71fbd4baecc717405318
Reviewed-on: https://go-review.googlesource.com/2808


Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>