Introduces a vk::Context class to contain an error handler and Renderer
pointer. This abtracts the common code path for ContextVk + DisplayVk.

Removes vk::Error in favor of the POD angle::Result class. There are a
few remaining usages of gl::Error that will have to be cleaned up when
we can change the front-end APIs.

Bug: angleproject:2713

Change-Id: I5e68f223d595c6c561b59d6a85759e5738ed43c6
Reviewed-on: https://chromium-review.googlesource.com/1128924
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>