gl::ErrorSet can handle errors and is accessible from ContextImpl.
It allows the implementation to mutate the error set without using
the gl::Context directly.

angle::Result is the faster POD return value class. It should generate
optimal code. It can also be used seamlessly with the ANGLE_TRY macro.

Also introduces an internal enum as a workaround for generating and
consuming errors in the back-end. When the internal enum is used as
an error return value the error is not consumed in the front-end. This
is a temporary workaround only.

Bug: angleproject:2491
Bug: angleproject:2713
Change-Id: I6cbdaadd075ccbdf241844cbcbc4ed5c3be40a8b
Reviewed-on: https://chromium-review.googlesource.com/1133200
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>