Allow custom callback_exception callable in ChatFeed

[ahuang11]

Users currently are limited to summary, verbose, ignore, and raise, which are all uncustomizable. Ideally, we should let users customize exception handling, e.g.

import panel as pn
pn.extension()

def callback_exception_handler(exc):
    ...

def callback(content, user, instance):
    1 / 0

pn.chat.ChatInterface(callback=callback, callback_exception=callback_exception_handler).servable()