Update Renderer11::packPixels to return Error objects.

BUG=angle:520
BUG=394361

Change-Id: I95f2f806210ac7aea8b0c57bece610eb22d018ed
Reviewed-on: https://chromium-review.googlesource.com/222481
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>