What should we do about `InvalidGenerationSourceError.todo`?

https://pub.dev/documentation/source_gen/latest/source_gen/InvalidGenerationSourceError/todo.html

Back in the day, I used to actually output errors to the target file with `// TODO: message`. We don't do that anymore.

This is heavily related to https://github.com/dart-lang/source_gen/issues/454, obviously.

In the short term, I could simply add todo content (it if exists) to the `toString` of `InvalidGenerationSourceError`.

I can also mark `todo` as deprecated.

Thoughts? @natebosch @davidmorgan @matanlurey @jakemac53

Start with the InvalidGenerationSourceError.todo API documentation and the current implementation, then read related issue #454 and the two comments for context. The scope is not decided: done requires an agreed policy for todo, such as retaining it, including it in toString, or deprecating it, together with updated API documentation and tests if behavior changes.

Written by the indexing model from the issue text.