magento/devdocs#: Add createEmptyCart mutation API errors (#5881)

- https://devdocs.magento.com/guides/v2.3/graphql/mutations/create-empty-cart.html

Author: atwixfirster

guides/v2.3/graphql/mutations/create-empty-cart.md

@@ -71,3 +71,10 @@ Attribute |  Data Type | Description
 ## Output attributes
 
 The `createEmptyCart` mutation returns the cart ID.
+
+## Errors
+
+Error | Description
+--- | ---
+`Cart with ID "XXX" already exists.` | The specified cart ID was previously used to create a cart.
+`Cart ID length should to be 32 symbols.` | The cart ID is not the required length.