Missing UUID fails run completely

If the generator encounters a UUID in the canonical data it crashes
instantly.

The user is then presented a stack that includes the offending UUID in
the `reason`.

This is not very userfriendly. Instead it should be prefered to ignore
the offending UUID as if it was set to `false` in the `tests.toml` and
generate a list in the background that would be presented to the user
at the end of the run.

Preferably in a way that makes it easily copy pastable into the
`tests.toml`.

The issue names no files or tests. Start by locating the canonical-data UUID handling and the final run-result formatting, then trace the current stack-producing path; done means missing UUIDs are skipped and reported at the end in copy-pastable tests.toml form.

Written by the indexing model from the issue text.