Added an egl::Error class and updated libEGL to use it.

BUG=angle:520

Change-Id: I792c8ddd8e8b76184f566294196d089bc9d1902a
Reviewed-on: https://chromium-review.googlesource.com/223270
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>