title: How do I use `std::error_category` and the other stuff that is in the system_error header?
body: <p>There are enough error handling strategies in C++ already. We have exception handling, error return codes and this <code>ERRNO</code> mess. What role does the <code>system_error</code> header play here? How do I use the features in there? To me, it just looks randomly put together. I'm using the <a href="http://en.cppreference.com/w/cpp/header/system_error" rel="noreferrer">cppreference</a> site as a reference. </p>