This is a small specialized macro to remove all the redundant
handleError calls and return value checking.

Also add a Context::prepareForDraw member that will be used in
follow-up patches to do necessary work prior to a draw call.

BUG=angleproject:2107
BUG=angleproject:2116

Change-Id: I8a32d2206c218fcca5236abfd3f2ce370296ca99
Reviewed-on: https://chromium-review.googlesource.com/585288Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>