Gmoshkin/proc custom error code

Summary

• feat: IntoBoxError for structured error info from stored procedures

• feat: implement getting error location from box_error_last

• feat: easier construction of BoxError

• refactor: remove tarantool mentions from error messages

• feat: add to_cstring & into_cstring helper functions

• refactor: use tarantool::error::Error in examples/async-h1

• chore: remove one redundant test

• fix: used to use the message as the format in tarantool::set_error!

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