Feature Request: add `class_error`, `class_condition`, `class_warning` base classes

[sebffischer]

It would be great if these were available, so it would be easier to use S7 to implement custom error classes.

[hadley]

Could you expand a bit on why you see the benefit of this? Condition objects are usually pretty simple.

[sebffischer]

When I initially created this issue I was working on a package where I wanted to just use S7 for everything and it just felt odd having to switch to S7 for condition objects. And whether condition objects are simple depends on the type of error handling you want to support for your users I guess and how complex your internal error handling is.
For my specific use case -- converting error messages from 0-based indices to 1-based indices -- the error classes are reasonably complex (I separate the content of the condition objects from its string representation) and S7 would have been a great fit for it, had it been available: https://github.com/r-xla/stablehlo/blob/main/R/conditions.R. I would expect that not covering condition objects in the S7 base package would entail that separate package developers will implement their own condition classes on top of S7, so having it in the base S7 package seems preferable to me.

[hadley]

I've made a PR so we can discuss it concretely, but something about it just doesn't fit quite right with me. Maybe because I'm not sure that we want S7 to provide wrappers for every existing S3 class in base R? Maybe because it feels like these wrappers don't add much value to me? Maybe because you still need to understand a bunch about the underlying list representation and what methods you need to define?

OTOH maybe this could be a place where we could provide more details for people who do want to extend condition classes for their own purposes?