[QA]: use nodiscard

[kitsudaiki]

QA

Description

With c++17 the [[nodiscard]] was introduced, which can be added to functions, so the compiler gives warnings, when the return-value of the function is not used. This is a quite nice feature to increase security and stability of the code itself, so it should be used where ever it is reasonable.

Acceptance Criteria

• all functions and methods in the code, with are used for checks or returning true/false to indicate success/fail, now have the [[nodiscard]] in front of it
• all resulting compiler-warnings are fixed, by adding checks for the return value

Additional Information

• https://en.cppreference.com/w/cpp/language/attributes/nodiscard

Blocked by

How to test

• compile code and check if there are still no warnings