Use `expected<monostate, Error>` instead of `optional<Error>`

These types are isomorphic to each other, but using `optional<Error>`
leads to an awkward API when checking for errors. The new type is a bit longer to spell out, but leads to a more intuitive API and simpler usage
by callers.

Change-Id: I73446077480ca3652d22d7ab2bae3435df7528c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4270073
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Shuran Huang <shuuran@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1111096}