Explain errors as data

As suggested in https://github.com/graphql/graphql-js/issues/560

Sometimes errors are totally expected and deserve to be represented as data in an API. This is worth a best practices post

Start by reading this issue and the referenced graphql/graphql-js#560 discussion to understand the proposed guidance. Identify the intended audience, examples, and location for a best-practices post; done should mean the agreed post is written and published in the website's documentation.

索引モデルが issue の本文から書いたものです。