Update the getRenderTarget functions to return gl::Error objects. BUG=angle:520 Change-Id: If1f4f71972b669704eff70b5f60927d8e6ac07b3 Reviewed-on: https://chromium-review.googlesource.com/218767 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>