Bootstrap: Changelog not displayed when config fails to parse #121756

Bootstrap's "changelog" functionality can be quite helpful in figuring out how to adapt a local config.toml file for bootstrap changes.

However, before checking whether the changelog needs to be displayed, bootstrap currently fully parses and validates the config. This means that if there is an error while parsing the config, the changelog is never displayed.

I ran into this recently when the codegen profile was removed (the profile must be successfully resolved to parse the config, so the changelog was never displayed), and again while writing #121754 (where I'd ideally like to remove a field from the config, but kept it for now to be able to provide a better error message).

IMO bootstrap would ideally check whether the changelog needs to be displayed before fully parsing the config.