8.2.7 to 8.5.1 broke my website

I tried updating my packaged distro of 8.2.7 straight to 8.5.1 and I ended up getting “The website encountered an unexpected error. Please try again later.”  I could for a while check my dblogs, for which there was nothing but now the whole site is unreachable.  There seems to be no way of knowing if the db was upgraded at all, or where the upgrade process got to.  Do I just have to start again with 8.27, with a new database and go through incremental upgrades or where have I gone wrong?  If I rerun the install.php I just get “Fatal error: Cannot redeclare twig_theme() (previously declared in /var/sites/n/mysite/public_html/core/themes/engines/twig/twig.engine:15) in /var/sites/n/mysite/public_html/core/core/themes/engines/twig/twig.engine on line 19″. I can’t access any of the site now.

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