register user errorhandler in the right order

solved the problem proposed by  #1291 in a lighter way,
thus determining the `exception  hierarchy` at app setup time instead of running time.
I did this intentionally .Because I use a totally new `list` instance to replace the original `self.error_handler_spec[key][key]` one, but since the new one is just based on the original one with some diff, it's should be OK to modify this 
without any break, don't you think so ?