Here's the link to the open bug, which is pretty recent too: https://www.drupal.org/node/2723707
How did I get here? I deleted the module, easychart, by removing the directory via FTP instead of uninstalling it "properly". Went to go clear the cache via drush and ended up with PHP fatal errors and the dreaded white screen of death (WSOD).
To fix this, you will have to hack Drupal core. I know, I know...don't ever hack Drupal core, but there's no other way around this if you want an up-to-date Drupal installation (8.1.1 as I'm writing). Go to https://www.drupal.org/node/2723707 and apply the patch there.
After patching, make sure you go into the database table, config, and remove ANY records associated with that module. In my case, I searched for easychart and removed any records associated with it.
After database cleanup, make sure to SSH in and perform a "drush entup" to update entities. This will prompt you to uninstall the leftover entity for that module. If there isn't one then you're done!
Keep an eye on that bug report and make sure to officially update Drupal when it gets patched!
Reminder: Always, ALWAYS make a backup of the database and files before updating core.