GL back-end error refactor. Adds explicit error handling to a few scoped state handler classes. Otherwise mostly mechanical refactoring. Bug: angleproject:2753 Change-Id: I2bf969a68f45880902b6e7902297c1340a76a1c4 Reviewed-on: https://chromium-review.googlesource.com/c/1255647 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>