Add a way to return an error from a calculation

[rbino]

As discussed on the Elixir forum, it would be nice to have a way to expose the failure of a calculation if it's fallible (e.g. if it queries an external API).

This way extensions that are able to do so (e.g. AshGraphql) can expose partial results alongside failures instead of just returning nil which doesn't convey the idea of a (possibly temporary) error.