Introduce error code to `BgtaskFailedEvent`, `BgtaskPartialSuccess`

## Motivation

As a client tracking `bgtask`, receiving only a list of error messages makes it impossible to handle different error types differently.
Let's introduce error codes into the `bgtask` result as well.

JIRA Issue: BA-2317

Start by locating the definitions and producers of BgtaskFailedEvent and BgtaskPartialSuccess, then trace how bgtask results are returned to clients. Define the error-code representation and update the result behavior so clients receive codes alongside error messages; done means both event types expose the new codes consistently.

Written by the indexing model from the issue text.