Convert session_save_path from error to warning. Determine if it even exists.

The php `session_save_path` thing was bugging me so I made some tweaks to the add-on to downgrade its importance from an error to a warning. Decisions were made based on [http://devot-ee.com/add-ons/support/health-check/viewthread/210](http://devot-ee.com/add-ons/support/health-check/viewthread/210).