[Bug]: it should error when there is no main config file

[vanessayuenn]

Describe the bug

Project: SvelteUI
Version: 8.0.0-alpha.13
Bug/feedback:

The upgrade command didn't warn/error about not being able to find the main configuration file. The configuration is in a non-standard directory which the user must define with the --config-dir flag, but there was no prompt for this. This caused all automigrations to be skipped.

see #25692

To Reproduce

No response

System

No response

Additional context

No response

[ndelangen]

It does actually warn the user, but I chatted with @shilman about this, and we decided to promote it to an error.

Here's what it does today:

I'll fix this inside of #26035