title: Custom condition for GraphQL mutation that returns message to user
body: <p>What would be the best way to create a response for the user when he is trying to add a new item to an order that is not longer in stock?
  Here is my simplified setup:</p>
  <pre><code>type Product {
      id: ID!
      unitsInStock: Int!
  }
  
  type OrderItem {
      id: ID!
      units: Int!
      product: Product! @belongsTo
  }
  
  type Mutation {
      createOrderItem(input: CreateOrderItemInput! @spread): OrderItem @create
  }
  
  input ProductBelongsTo {
      connect: ID
      create: CreateOrderItemInput
  }
  
  input CreateOrderItemInput {
      units: Int!
      product: ProductBelongsTo
  }
  </code></pre>
  <p>Basically, before committing the <code>createOrderItem</code> mutation I want to check if the product selected for the order is present in stock, for example, if the user selects too many units of the product he would get a friendly message saying &quot;only 10 items left remaining&quot;, or if stock is depleted &quot;this item is no longer in stock, please update stock to continue&quot;.</p>
  <p>How can I do that?</p>