Framebuffer: Handle errors in checkStatus. This pipes a lot more errors around in the Validation, where they now will be caught. Bug: angleproject:2372 Change-Id: Ibb4e47ddc932995a02dd92e10578b7a4097182a9 Reviewed-on: https://chromium-review.googlesource.com/954406 Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>