Code review comment for lp:~mvo/ubuntu-release-upgrader/use-str.format

Maybe str.format is better for other reasons, but you can certainly still get tracebacks from it:

$ python3 -c "'hello {0'.format('world')"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: expected '}' before end of string