Upgrade to SQL Server 2017 Fails
Here is what resolved the error, copied lovingly from a Microsoft forum 

Here is what I'll be trying in a bit (from here)

1) Execute Diff backup in SQL Safe
2) Create Snapshot in VCD
3) Perform Upgrade
4) When fails add -T902 flag to startup parameters
5) Start SQL Server
6) Install CU 11
7) Remove -T902
8) Start SQL Server


The key here was to start up sql server with the -T902 parameter and then install the latest CU (which is 11 right now)