Added the error coverage for cart query

src/guides/v2.3/graphql/queries/cart.md

@@ -790,3 +790,11 @@ Attribute |  Data Type | Description
 *  [setShippingMethodsOnCart mutation]({{page.baseurl}}/graphql/mutations/set-shipping-method.html)
 *  [setBillingAddressOnCart mutation]({{page.baseurl}}/graphql/mutations/set-billing-address.html)
 *  [setPaymentMethodOnCart mutation]({{page.baseurl}}/graphql/mutations/set-payment-method.html)
+
+## Errors
+
+Error | Description
+--- | ---
+`Could not find a cart with ID \"xxxxx\"` | The ID provided in the `cart_id` field is invalid or the cart does not exist for the customer.
+`The cart isn't active` | The cart with the specified cart ID is unavailable, because the items have been purchased and the cart ID becomes inactive.
+`Field cart.cart_id of required type String! was not provided` | The value specified in the `cart.cart_id` argument is empty.

src/guides/v2.4/graphql/queries/cart.md

@@ -829,3 +829,11 @@ Attribute |  Data Type | Description
 *  [setShippingMethodsOnCart mutation]({{page.baseurl}}/graphql/mutations/set-shipping-method.html)
 *  [setBillingAddressOnCart mutation]({{page.baseurl}}/graphql/mutations/set-billing-address.html)
 *  [setPaymentMethodOnCart mutation]({{page.baseurl}}/graphql/mutations/set-payment-method.html)
+
+## Errors
+
+Error | Description
+--- | ---
+`Could not find a cart with ID \"xxxxx\"` | The ID provided in the `cart_id` field is invalid or the cart does not exist for the customer.
+`The cart isn't active` | The cart with the specified cart ID is unavailable, because the items have been purchased and the cart ID becomes inactive.
+`Field cart.cart_id of required type String! was not provided` | The value specified in the `cart.cart_id` argument is empty.