Most of the time people are interested in successful executions. Unfortunately,
unwrap package can't help here because of a different result structure (some
interface abstract can help, but it's still mostly stack-oriented and sessions
can be a problem), so this additional interface is needed.
Signed-off-by: Roman Khimov <roman@nspcc.ru>
Somewhat similar to invoker, but changing the state (or just creating a
transaction). Transaction creation could've been put into a structure of its
own, but it seems to be less convenient to use this way.
