This cleans up some messy stuff from the emulated index buffers, which
were caching variables that didn't need to be cached.

Also add in missing error checks. This touches a lot of code.

BUG=angleproject:1327
BUG=angleproject:1310

Change-Id: Icd722d57d9449388fbabc62c7ea37f0526a568ff
Reviewed-on: https://chromium-review.googlesource.com/334731Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>