Gmoshkin/iproto client connection closed error

Summary

• refactor: rework what errors are returned when working with network::Client

• fix: mark error enums as #[non_exhaustive]

Needed for picodata!855 (merged)

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files