Draft: Adding guide lines to customize GraphQL errors

What does this MR do and why?

Our current GQL back-end style guide has no topic on the subject of error handling. It is mostly handled by just passing a human-readable message to a GQL error instance just like here.

Sometimes developers need to customize their errors for various reasons like it was the case with #343260 (closed).

This guide lines MR show them how to do so according to the GraphQL specification and the graphql-ruby gem documentation.

Closes Style guide for GraphQL error handling proposal (#384266)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

closes #384266