message: Support overriding of error renderers
diff: @@ -353,7 +353,7 @@ actor_system_config::add_actor_factory(std::string name, actor_factory fun) {
  actor_system_config&
  actor_system_config::add_error_category(atom_value x, error_renderer y) {
  - error_renderers.emplace(x, y);
  + error_renderers[x] = y;
  return *this;
  }
  