Also some minor fixes to validation and error messages.

BUG=angleproject:747

Change-Id: I4f97a45c2d39a8deec2255620e5cc2bcb8cad7b9
Reviewed-on: https://chromium-review.googlesource.com/637126Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>