Move ReadPixels error generation out of the implementation and into
the API level.

BUG=angle:571

Change-Id: I0b32294f359fedd13d1af2c95baf37a3e5ac1d5b
Reviewed-on: https://chromium-review.googlesource.com/188014Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>