Narrow contract rvalue reference observer of the stored error. NoValuePolicy::narrow_error_check() is first invoked, then the reference to the error is returned. As a valid default constructed error is always present, no undefined behaviour occurs unless NoValuePolicy::narrow_error_check() does that.

Note that if error_type is void, only a const overload returning void is present.

Requires: Always available.

Complexity: Depends on NoValuePolicy::narrow_error_check().

Guarantees: An exception is never thrown.