message: utils: Support empty collection in `pretty_listing`.
diff: @@ -79,6 +79,8 @@ def pretty_listing(c, any = False, quantifier = None):
  '''
  Format a collection of items into a human readable listing.
  + >>> pretty_listing([])
  + 'none'
  >>> pretty_listing(['foo'])
  'foo'
  >>> pretty_listing(['foo', 'bar'])
  @@ -98,7 +100,9 @@ def pretty_listing(c, any = False, quantifier = None):
  c = list(c)
  elif not isinstance(c, (list, tuple)):
  return str(c)
  - if len(c) == 1:
  + if len(c) == 0:
  + return 'none'
  + elif len(c) == 1:
  return str(c[0])
  return '{}{} {} {}'.format(
  '{} of '.format('any' if any else 'all') if quantifier else '',
  