Running the following script prints "Unhandled Error", but doesn't
contain the traceback of where the exception is raised. In a massive
codebase, it's hard to locate where is the malfunctioning code is.
from twisted.internet.defer import Deferred
d = Deferred()
# how to show the traceback without manually adding an errback at the end?
# adding at the *end* of a deferred is hard as we don't know when the
client stops adding callbacks.