[FFe] Autoinstall Failures feature

[Chris Peterson]

I would like to request a Feature Freeze exception for the feature “Improved Presentation of Autoinstall Failures” in Subiquity.

Autoinstall issues are a common point of confusion for users and are often difficult to troubleshoot. In the various fully vs partially automated install cases, on both desktop and server, the way we present schema validation failures or similar runtime checks is not terribly clear to the user running the install. Improving our presentation of failures will improve user experience. Additionally, we hope to reduce the number of bugs reported against subiquity because a bad autoinstall definition wasn’t caught.

Some examples of the feature:
- Comprehensive validation on autoinstall top level keys
- Clear indication of failure source at run time
- Detecting autoinstall incorrectly sent over cloud-config
- A specific AutoinstallException for common autoinstall errors
- Disable the automatic apport report generation for known cases of user error (e.g., a bad autoinstall definition).
- Improved logging to the serial console in the fully automated install case
- A new screen in the TUI for the partially automated install case

The feature is not yet complete, but is on track to be committed to the Subiquity Main branch within the following weeks, which should still allow time for extensive testing. I have an open PR here:

https://github.com/canonical/subiquity/pull/1925

which contains most of the large architectural changes required for the feature (e.g., changes to exception handling, API changes, a new screen in the server TUI, etc.).

[Łukasz Zemczak (sil2100)]

This is fine. Please be sure to finalize this before beta for sure. Exception granted.