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/1255647Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>