This simplifies the error handling code, and allows the use of the
ANGLE_TRY macro (and friends).

BUG=angleproject:1576

Change-Id: I3142388b10d0dd67a7f49a5c8c3d11102996be93
Reviewed-on: https://chromium-review.googlesource.com/411201
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>