Proposal: add support for warnings

[per-samuelsson]

That's something we wanted for long. I guess it's really about being able to introduce a code for anything, and then bind it to a severity level (i.e. warning, error).

Since we might want to change terminology of classes when doing that, I suppose we should do it at some point where we do other changes which will require us to update a big set of files in batch, like proposed Error.SCERRUPPERCASE => Error.ScErrPascalCase, and then do change maybe to Issue.ScErrWarningOrErrorOfSomeKind.