Top level error shape improvements (#172)

* Fallback to a generic error representation for unmodeled errors

* DRY out some dependencies

* Fix name is instant_iso8601 resource

* Top level error shape improvements

This commit adds `code` and `message` as inherent methods to top level error shapes (eg. `ListTablesError`). If the error is unmodeled, it uses
downcasting to extract a message.

* Fix clippy warning