This change makes errors in the example code a bit better, as it's no use to show the root dir when an error occurs walking a subdirectory or file.

Change-Id: I546276e9b151fabba5357258f03bfbd47a508201
GitHub-Last-Rev: 398c1eeb6164a7edc6fdee8cb8c17c3bd0b649ef
GitHub-Pull-Request: golang/go#24536
Reviewed-on: https://go-review.googlesource.com/102535Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>