Add logging in validation handler for validation errors
Add logging in validation handler for validation errors

Bug: 761488
Change-Id: I9518ce3431eb4f6d9d0eee2d58125235ee82859f
Reviewed-on: https://chromium-review.googlesource.com/830275
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Mun Yong Jang <cafb0219fa074fe0c089777a22a0dd4cc2126d7e@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go