`Outcome<void, T>` and `Result<std::optional<T>>` can be interpreted as
being the same thing, but the latter is easier to use: not only do they
allow TRY_AWAIT usage for their promises, we also don't have the error/
exception confusion of outcomes (where the T above is the "error" type)
Change-Id: I92c9241481cecc97e2992445b3dced53c82a2524
eldritch horrors
b43d7b8136