Fix saga import; small refactoring and additional comment to make the workflow more readable

In the newest releases of QGIS, SAGA was moved from processing.saga to processing.algs.saga, therefore we needed to change the import.
The method checking if SAGA is installed returns a message in case of error, or None otherwise (a bit counter-intuitive), so I added a comment and I made a small refactoring to make it clearer.