(DOCSP-17587): update sqlgenerateschema error message

Author: jwilliams-mongo

source/reference/cli/sql/sqlgenerateschema.txt

@@ -304,7 +304,12 @@ The command returns the following error if the command fails:
 .. code-block:: none 
    :copyable: false
 
-   failedNamespaces: [<db.ns>]
+   "failedNamespaces": [
+     { 
+       "namespace" : "<db.ns>", 
+       "error" : "no documents found in sample namespace" 
+     }
+  ]
 
 The above error is returned if the specified namespaces do not 
 exist in the storage configuration or are empty. This error is