This pipes errors up from the Impl to the top level. There are
still a few places were error swallowing is needed, because the
Framebuffer API doesn't support returning an error.

Bug: angleproject:2372
Change-Id: Idc06bda1817fd28075940f69874d8b6ba69194f9
Reviewed-on: https://chromium-review.googlesource.com/954290
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>