message: Add [[nodiscard]] attribute
  Add [[nodiscard]] attributes to the forward declarations
  of error and error_code in fwd.hpp
diff: @@ -33,7 +33,7 @@ namespace caf {
  template <class> class behavior_type_of;
  template <class> class dictionary;
  template <class> class downstream;
  -template <class> class error_code;
  +template <class> class [[nodiscard]] error_code;
  template <class> class expected;
  template <class> class intrusive_cow_ptr;
  template <class> class intrusive_ptr;
  @@ -106,7 +106,7 @@ class config_value;
  class deserializer;
  class downstream_manager;
  class downstream_manager_base;
  -class error;
  +class [[nodiscard]] error;
  class event_based_actor;
  class execution_unit;
  class forwarding_actor_proxy;
  