Updated mipmap generation to return Error objects.

BUG=angle:520

Change-Id: Ic4e57148d031d6c452b3054efad98f6e730c7691
Reviewed-on: https://chromium-review.googlesource.com/221394
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
Updated Image11 to use gl::Errors for staging texture manipulation.

BUG=angle:520

Change-Id: I5792b139ddd69c2cabc3b901a7067438c1ee363b
Reviewed-on: https://chromium-review.googlesource.com/221395
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>