[workspace] Examine `Option` / `bool` return types

[clabby]

Overview

In a few areas around the codebase, we're returning Options and bools for functions that deserve a typed error. We should examine the interfaces of mailboxes, etc. to find places where we should add descriptive errors.

Note that all Errors are Debug + Display, so these can still be type-erased to Strings to avoid generic hell in thiserror-style error nesting.