CLI: Refactor custom error structures

[avahahn]

Currently both unit-client-rs and unitctl use a custom error crate that is not only superfluous but also hard to read. It is reducing overall code quality. Removing it and implementing a non-external solution would lead to quicker, smaller builds and more readable code.

Unitctl (bearable): https://github.com/nginx/unit/blob/master/tools/unitctl/unitctl/src/unitctl_error.rs Unit-client-rs (abject disaster): https://github.com/nginx/unit/blob/master/tools/unitctl/unit-client-rs/src/unit_client.rs