We would traverse the applyRenderTarget path from several entry
points, which could then trigger a GL error. In-line with our
validation refactoring, move all error checks prior to executing
any state logic.

BUG=angle:571

Change-Id: I51f5bcfa41be7cdba7771eb87d5a831fa622f984
Reviewed-on: https://chromium-review.googlesource.com/203772Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>