reader: move error check in validate
reader: move error check in validate

The method can report the error and return
a bit earlier if there is a problem.

Change-Id: Icf6c258cb0f994cebc7a717e1e28e7b6ec95dd60
Reviewed-on: https://go-review.googlesource.com/15908
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

shuLhan/tour,Go-zh/tour,dupoxy/go-tour-fr,zorion/go-tour-ca,scryner/tour,Go-zh/tour,shuLhan/go-tour-id,shuLhan/go-tour-id,shuLhan/go-tour-id,scryner/tour,golians/tour,Go-zh/tour,zorion/go-tour-ca,golians/tour,golang/tour,dupoxy/go-tour-fr,scryner/tour,shuLhan/tour,Go-zh/tour,dupoxy/go-tour-fr,shuLhan/tour,scryner/tour,golians/tour