Retrieves a registry value for a specified key.
Returns an error message or an empty value, depending on return-empty-value.
The required registry-key-value is the name of the key.
The optional return-empty-value determines what is returned if the registry key is not found. If it is not passed, or it is passed with the default 'No' value, an error message is returned. If it is passed with the 'Yes' value, an empty value is returned.