Expose sub-crate *Error types at top-level.
Expose sub-crate *Error types at top-level.

We're not exposing a uniform API with `mentat::Result` yet, meaning
that early consumers (e.g., the logins work for Mozilla Lockbox) need
to wrap errors from all over the Mentat crate hierarchy.

mozilla/mentat,ncalexan/mentat,ncalexan/mentat,ncalexan/mentat,ncalexan/datomish,ncalexan/mentat,ncalexan/datomish,ncalexan/mentat,mozilla/mentat,mozilla/mentat,mozilla/mentat,mozilla/mentat,ncalexan/mentat,mozilla/mentat