Revert get_error_handler

Author: redboltz

include/mqtt/callable_overlay.hpp

@@ -2019,7 +2019,7 @@ struct callable_overlay final : public Impl
      * @brief Get error handler
      * @return handler
      */
-    error_handler const& get_error_handler() const {
+    error_handler get_error_handler() const {
         return h_error_;
     }