"Source database does not contain a recognizable Drupal version."
At this stage, I don't think there is a simple option to upgrade from 7 to 8. As you can see on the release note: 


  Once you are ready, Drupal 8 core also includes the Migrate module to
  update existing Drupal 7 and 6 sites to Drupal 8 directly. Migrate is
  marked "experimental" in Drupal 8.0.0, but will be fully supported in
  an upcoming release.
  https://www.drupal.org/news/drupal-8.0.0-released


A little bit technical behind the scene: 
From 7 to 8 version, they keep the same concept when building site (like node, entity, permission, views...) but not the core. I would say: they changed everything to OOP, Symfony component, architecture... 
So there is no way to upgrade your drupal site directly from decent version to 8.0, you have to migrate. Here is how the migrating process should look: 


Recreate the site with the same functionality to your d7 site. 
Recreate the theme (using twig template)
Migrate content over 


The cost to this process is (unfortunately) same to recreate a new site or more. 
With the no 3, take a look at this article by Phase 2: https://www.phase2technology.com/blog/upgrading-to-a-drupal-8-site/