Warnings represent resumable exceptional conditions that are non-fatal.
Resuming after an unhandled warning is usually successful, though the user normally has the option of cancelling the operation (though this will depend on the session manager, which once again is notified of unhandled warnings via the #onUnhandledWarning: message).
In the development system (when the DevelopmentSessionManager is installed) the user also has the option of "retrying" after a warning, which brings up a walkback allowing one to enter the debugger.
The most general form of warning is represented by the class Warning, which like Error contains a displayable description. One does not normally need to subclass Warning, although it is permissible to do so.