Forbid raising errors in GraphQL object types and resolvers

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We discourage raising errors in graphql resolvers (see https://gitlab.com/gitlab-org/gitlab/blob/master/doc/development/api_graphql_styleguide.md#L977-981). We should enforce this with a rubocop rule since this will cause 500 errors and fail entire queries, potentially leaking information and causing mutations to fail that would otherwise succeed.