Improve validation of Gen/Delete calls Add checks for negative count to GenTransformFeedbacks and DeleteTransformFeedbacks, and check for active transform feedbacks in DeleteTransformFeedbacks. Unify validation and error messages of all other Gen/Delete calls. BUG=angleproject:1101 TEST=dEQP-GLES3.functional.negative_api.* (two more tests pass) Change-Id: I128063fab3db27a25e282a10c916c53646d68b9c Reviewed-on: https://chromium-review.googlesource.com/332142 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>