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/332142Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>