[[nodiscard]] for ntsa::Error

I was thinking if it can be useful for us to declare `ntsa::Error` as `class [[nodiscard]] ntsa::Error` when the compiler supports this feature.

At the moment such declaration results in a lot of compilation warnings, mainly in test drivers. But as the idea of ntsa::Error is to return an error status from a function it's better to not ignore it.

Start by locating the declaration of ntsa::Error and reviewing the compilation warnings it causes in test drivers. Check how compiler feature support is handled, then verify that supported builds diagnose ignored errors without leaving unintended warnings; done means the declaration is applied consistently and affected tests compile cleanly.

Written by the indexing model from the issue text.