Update our internal GraphQL Styleguide documentation with information about errors in mutations (and how to test them)

See relevant discussion here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29919#note_184446765 about the difference between raising exceptions and returning an errors response from the mutation.

There are also two new shared spec helpers in that MR that let us test error responses in /spec/support/shared_examples/graphql/mutation_shared_examples.rb/

We should update our GraphQL Styleguide document with this information.