Refactored etl::error_handler to use etl::delegate style implementation.
Refactored etl::error_handler to use etl::delegate style implementation.

Allows set_callback() function to be given run-time and compile-time pointers to free and member functions without using etl::ifunction.

ETLCPP/etl,ETLCPP/etl,ETLCPP/etl,ETLCPP/etl