Hmm.
A program should stop with an error when it encounters an error condition (a) which is unexpected, (b) it doesn't know how to recover from, and (c) action by the user is probably necessary to fully resolve (in fact, (c) is a bonus; (a) and (b) are probably sufficient to justify stopping). So I don't think apt-listchanges is wrong to exit here. Perhaps, though, it should just print an error message and exit with a non-zero exit status rather than generating a Traceback.
What do you think?