I tried to install SQL Server Express 2017 and something went wrong during the installation process. As it turns out, that might have been an oversight on my part, but the result was that the process was rudely terminated.

Now I'm in limbo. I can't repair the installation, I can't uninstall...and I can't think of anything else to do. Uninstall hangs forever on this step:

SqlEngineConfigAction_remove_Validation_Cpu64

I have left it sitting on that step for fifteen minutes, or so, and it never moves from there. Repair doesn't hang, it just fails when attempting to repair the database engine with this message in the log:
The SQL Server feature 'SQL_Engine_Core_Inst' is not in a supported state for repair, as it was never successfully configured. Only features from successful installations can be repaired. To continue, remove the specified SQL Server feature.
Interestingly, I can go ahead and install the default instance and uninstall the default instance, so it looks like I could simply disable the bad instance and work around it, but that seems like a bad idea overall.

Can anybody tell me how to get rid of this thing or fix it so that it works?