Answers a thrown error: the Kernel's onError.
Answers with a problem the caller built (a failed validation, a missing
file). Only the contract's log sees it: the caller logs what it must.
Optionalerror: unknownThe problem for an error: the contract's toProblem, else Iskra's mapping, with the request id.
Answers requests by the contract: the Kernel's error handler and 404, and the
ok(),list()andfail()helpers, which find it on the context.