Consistent error handling

- [x] Server must return errors that have consistent format.
- [x] Unify way of handling with Throw and Callback errors.
- [x] Handle all errors. Now app can skip some errors, for example in PulsarDB.
Do you mean to write `callback(null, tasks);` instead of `callback(tasks);`? It is just that we don't have here any non-thrown errors.