Improve interoperability with std::system_error

[apolukhin]

While migrating Boost.DLL to std::error_code boostorg/dll#76 I've noticed that boost::system_error is not related to std::system_error. Because of that, users have to duplicate the catch blocks if there's a need to work with error code:

try {
  some_throwing_function();
} catch (const std::system_error& err) {
  process_error_code(err.code());
} catch (const boost::system::system_error& err) {
  process_error_code(err.code());
}

With this PR the seconds catch block could be eliminated. Is there an interest in such functionality? The output of e.what() changes, is that OK?

[pdimov]

No, it's not OK, because it throws away information, namely the source location associated with the error code. E.g. https://godbolt.org/z/Tcxf8TdY9.

I assume you don't care about this because you never added source locations to your error codes, but others have. :-)

This will also change the behavior of code like the one you show - the first catch block will start catching our system_error and the second one will never be reached.