Add error handling macro to Context. 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/585288 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>