Vulkan: Use angle::Result error handling. 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>